The Evolution Theory, Vol. 2 of 2

by August Weismann

LECTURE XXVII

THE BIOGENETIC LAW

Fritz Müller's ideas—Development of the Crustaceans—Of the Daphnidæ—Of Sacculina—Of parasitic Copepods—Larvæ of the higher Crustaceans—Change of phyletic stages in Ontogeny—Haeckel's Fundamental Biogenetic Law—Palingenesis and Cœnogenesis—Variation of phyletic forms by interpolation in a lengthened Ontogeny—Justification of deductions from Ontogeny to Phylogeny—Würtemberger's series of Ammonites—Phylogeny of the markings in the caterpillars of the Sphingidæ—Condensation of Phylogeny in Ontogeny—Example from the Crustaceans—Disappearance of useless parts—The variation of homologous parts, according to Emery—Germ-plasmic correlations—Harmony with the theory of determinants—Multiplication of the determinants in the course of the phylogeny.

What I propose to discuss in this lecture should have been considered at an earlier stage, if we had pledged ourselves to adhere strictly to the historical sequence of scientific discovery, for the phenomena which we are about to deal with attained recognition shortly after the revival of the evolution idea, and indeed they formed the first important discovery which was made on the basis of the Darwinian Doctrine of Descent. I have introduced them at this stage because they have to do with phenomena of inheritance and modifications of these, the understanding of which—in as far as we can as yet speak of understanding at all—is only possible on the basis of a theory of inheritance. Therefore, in order to examine these phenomena and their causes, it was necessary first to submit a theory of heredity, as I have done in the germ-plasm theory. We have to treat of the connexion between the development of many-celled individuals and the evolution of the species, between germinal history and racial history, or, as we say with Haeckel, between ontogeny and phylogeny.

Long before Darwin's day individual naturalists had observed that certain stages in the development of the higher vertebrates, such as birds and mammals, showed a likeness to fishes, and they had spoken of a fish-like stage of the bird-embryo. The 'Natural Philosophers' of the beginning of the nineteenth century, Oken, Treviranus, Meckel, and others, had, on the basis of the transmutation theory of the time, gone much further, and had professed to recognize in the embryonic history of Man, for example, a repetition of the different animal stages, from polyp and worm up to insect and mollusc. But von Baer afterwards showed that such resemblances are never between different types, but only between representatives of the same general type, e.g. that of Vertebrata; and Johannes Müller maintained, from the standpoint of the old Creation theory, that an 'expression of the most general and simple plan of the Vertebrates' recurred in the development of higher Vertebrates, giving as an instance that, at a certain stage of embryogenesis, even in Man, gill-arches were laid down and were subsequently absorbed. But why this 'plan' should have been carried out where it was afterwards to be departed from remained quite unintelligible.

An answer to this question only became possible with the revival of the Theory of Descent, and the first to throw light in this direction was Fritz Müller, who, in his work Für Darwin, published in 1864, interpreted the developmental history of the individual, 'the ontogeny,' as a shortened and simplified repetition, a recapitulation, so to speak, of the racial history of the species, the 'phylogeny.' But at the same time he recognized quite clearly—what indeed was plain to all eyes—that the 'racial history' cannot be simply read out of the 'germinal history,' but that the phylogeny is often 'blurred,' on the one hand by the fusing and shortening of its stages, since development is always 'striking out' a more direct course from the egg to the perfect animal, while, on the other hand, it is frequently 'falsified' by the struggle for existence which the free-living larvæ have to maintain.

For the establishment of these views Fritz Müller relied chiefly upon larvæ, and in particular upon those of Crustaceans, and the facts, which were in part new and in part interpreted in a new manner, were so striking that it was impossible to deny their importance. In particular, he drew attention to the fact that in several of the lower orders of Crustaceans the most diverse species have a similar form when they leave the egg, all of them being small, unsegmented larvæ, with a frontal eye and a helmet-like upper lip, and with three pairs of appendages, the two posterior pairs being two-branched swimming-legs beset with bristles. In the size and form of the body, and especially of the chitinous carapace, these larvæ differ in the various systematic groups; thus, for instance, the larvæ of the Copepods are simply oval, while those of the Cirrhipedes are produced anteriorly into two horn-like processes, and so on, but in essentials they are all alike, and for a long time these larval forms had been distinguished by the special name of 'Nauplius' ([Fig. 109]).

The development of the perfect animal begins with the longitudinal growth of the Nauplius; the posterior end lengthens and becomes segmented, between the anterior portion and the tail more segments are interpolated, and on these new pairs of limbs may grow. The number of these segments and limbs varies according to the group to which the animal belongs. Thus the body of the perfect animal in the little Cyprids always consists of eight segments, seven of which bear a pair of limbs apiece; in the Branchiopods, on the other hand, the number of segments varies from twenty to sixty, with ten to over forty pairs of legs; in the Daphnids or water-fleas there are about ten segments, with seven to ten pairs of limbs, and in the Copepods about seventeen segments with eleven pairs of limbs. The difference between the orders depends not only upon the differences in the number of segments and limbs, but quite as much upon the form and development of the segments, and above all of the limbs, and in this connexion it is worthy of note that the additional limbs which grow out usually appear at first as biramose swimming-legs, and are subsequently modified in form. Thus the pairs of jaws, three in number, which appear in the Copepods are developed from such swimming-legs, and so also is the second pair of antennæ in the Copepods and the jaws of the Branchiopods, Cirrhipedes, &c.

Fig. 108. Nauplius larva of one of the lower Crustaceans. After Fritz Müller. Au, the frontal eye; I, first pair of limbs, corresponding to the future antennæ; II and III, two biramose swimming appendages.

If then we have before us in the 'germinal history' (ontogeny) a fairly precise repetition of the 'racial history' (phylogeny), we may deduce from this that the primitive forms of the Crustacean race were animals which consisted of few segments, and that from these, in the course of the earth's history, the very diverse modern groups of Crustaceans have arisen, by the addition of new segments, and the adaptation of the limbs upon them, which were at first biramose swimming-legs, to different kinds of functions, one becoming an antenna, another a jaw or a swimming-arm, a third, fourth, fifth, and so on, a jumping-leg, a copulatory organ, an egg-bearer, a gill-bearer, or a tail-fin.

Fig. 109. Metamorphosis of one of the higher Crustacea, a Shrimp (Peneus potimirim), after Fritz Müller. A, the nauplius larva with the three pairs of appendages: I, the antennæ; II and III, the biramose swimming-feet. Au, the single eye. B, first Zoæa stage, with six pairs of appendages (I-VI). Skn, area where new segments are being formed.

That the development has in general followed those lines is made clear chiefly by the fact that the members of all these different orders of Crustaceans still arise from nauplius larvæ, even in those cases in which the perfect animal possesses a structure differing widely from the usual Crustacean form. All Crustaceans arise from the nauplius form, even those of the higher orders, though they may not arise from a nauplius larva. But this very circumstance, that in most of the higher and many of the lower Crustaceans, the young animal, when it emerges from the egg, already possesses more numerous segments and limbs than a nauplius larva, again points to the connexion between phylogeny and ontogeny, for in these cases the nauplius stage is gone through within the ovum. The whole difference between this and the forms we considered first lies in the fact that, in the latter, the development is greatly shortened, condensed, as we might say, so that the nauplius stage forms a part of the embryonic development, and that new segments and limbs develop in the embryo nauplius within the egg, so that the young animal leaves the egg in a more advanced state, nearer to that of the perfect animal, to which it can, therefore, attain in a shorter time.

Fig. 109. C, second Zoæa stage. The
thorax is now divided into cephalothorax
(Cph) and abdomen (Abd); seven pairs of
appendages are developed, and five more
(VIII-XII) are beginning to appear. Au,
paired eyes.

We should expect that this shortening of the larval period would be associated with a prolongation of embryogenesis, especially in those Crustaceans which possess a large number of segments and limbs, that is—in the higher forms—and in the main this is the case. But there are exceptions in two directions; in the first place there are some, even among the lower Crustaceans, which leave the egg not as a nauplius but in the perfect form of the adult, and secondly, there are, among the higher Crustaceans, certain species which emerge from the egg not in the more mature form but still in the primitive nauplius form. Fritz Müller was the first to furnish an example of this last case, a Brazilian shrimp, Peneus potimirim. Like the lowest Copepods or Branchiopods, this species, which belongs to the highest order of Crustaceans, goes through the whole long development, from the nauplius through a series of higher larval forms up to the perfect animal, and all outside of the egg, as an independent free-swimming larva (Fig. 109, A-E). This is in sharp contrast to its near relative, the freshwater crayfish, which goes through this whole development within the egg, and emerges perfectly formed.

We see from this example that it is not some inward necessity which thus, in the higher and more complicated organism, contracts the ontogeny into the embryonic state, but that this depends upon external adaptive factors. Here again we have adaptation, mainly to the conditions of larval life. The elimination of the larvæ by enemies, for instance, will, other things being equal, be so much the more incisive the longer the larval development is protracted, but in that case the general ratio of elimination of the species, and the degree of fertility the species must possess in order to hold its own in the struggle for existence, will also play a part in determining the mode of development. For the higher the ratio of elimination the more eggs the female must produce, and the more eggs that have to be produced the smaller will be the quantity of nutritive material for the building up of the young embryo which each egg can be furnished with. I know of no records in regard to the eggs of that Brazilian shrimp in which embryonic development ends with the nauplius stage, but we shall certainly not be wrong in predicting that the eggs in this case will be very small and very numerous, in contrast to those of the freshwater crayfish, which are large and, as compared with others known to us, not very numerous.

It is a point of undeniable theoretical significance which the life-histories of these Crustaceans disclose, that embryogenesis is not condensed according to hidden internal laws when the structure increases in complexity, but that the condensation of the ontogenetic stages depends upon adaptation, and may be quite different in nearly related species. It shows us anew that all biological occurrences are dominated by the process of selection.

Fig. 109. D, Mysis-stage. Thirteen pairs of appendages are now formed: I and II, antennæ; III, mandibles; IV and V, maxillæ; VI-XIII, swimming appendages with one branch or with two. Abd, abdomen. Sfl, tail-fin. E, the fully-formed Shrimp, with thirteen pairs of appendages on the cephalothorax (Cph); I and II, the two pairs of antennæ; then follow the maxillæ and maxillipedes (III-VIII), the last of which is visible in the figure, and the five pairs of walking-legs (IX-XIII) of which the third bears a long chela. On the abdomen there are now six pairs of appendages (XIV-XIX).

I have already mentioned that exceptions to the usual mode of development occur even among the lower Crustaceans, and I was thinking at the time of the Daphnids, which leave the egg as fully formed little animals, already equipped with all their segments and limbs. The nauplius stage is passed through in the egg, and it is an interesting indication that the ancestors of the modern species were in the way of moulting, that this embryo nauplius moults within the egg by forming a fine cuticle which is shed after a time. If it be asked why there should be direct development in the case of these small and not very complex water-fleas, while related species, the Branchiopods, which are much richer in segments and in limbs, should emerge from the egg in the form of a nauplius, and then pass through a longer larval period, we may answer that the reason probably lies in the fact that, in the former case, very few eggs are produced, sometimes only one, often two, seldom more than a dozen, that these eggs can thus be relatively well equipped with yolk, and that the formation of the little body which bears only from seven to nine pairs of limbs can be easily completed within this egg. Other things being equal, the direct development would always be an advantage, because reproduction can begin sooner in the young generation and the number of individuals will thus increase more rapidly. And this is of particular importance in the case of the water-fleas.

But if it be asked, further, why so few eggs are produced in this case, and whether these animals have no enemies, we must answer that, on the contrary, they are preyed upon and eaten in thousands by fishes and other freshwater animals, but that the drawback of the scanty production of eggs is counteracted on the one hand by their habit of reproducing parthenogenetically for the greater part of the year, and on the other hand by their habit of concealing the eggs in a special brood-chamber. This is the case not only in the summer eggs, to which nourishment is conveyed in the brood-chamber from the blood of the mother (Fig. 70), but also in the winter or 'lasting' eggs, which receive within the chamber a protecting covering (the shell or ephippium).

Fig. 70 (repeated). Daphnella.
A, summer ovum, with
an oil-globule (Oe). B, winter
ovum.

In almost all the Daphnids the winter egg develops into a perfect animal just like that to which the summer egg gives rise, although it no longer receives any nourishment after it passes into the brood-chamber. But it receives a larger supply of yolk on this account, so that the nutritive provision within the egg is sufficient to develop the perfect animal. There is only one exception to this, and it is of special theoretical interest, because it shows more plainly than any other fact that the greater or less degree of condensation in the ontogeny depends upon the combined effect of the external conditions of life. The largest of the Daphnidæ, Leptodora hyalina, a beautifully transparent inhabitant of our lakes, which measures about a centimetre in length (Fig. 110), also emerges from the summer egg as a perfect animal, but from the winter egg, which floats freely in the water and has only a small provision of yolk, it emerges as a nauplius, which then undergoes larval metamorphosis before it becomes a perfect animal (Fig. 111).

Fig. 110. The largest of the Daphnids (Leptodora hyalina), with summer ova (Ei) beneath the shell (Sch). I-IX, the appendages. II, the oars (second antennæ) which always remain biramose in Daphnids. sb, setæ. ov, ovaries. Schl, œsophagus. Ma, stomach. a, anus. H, heart. Au, eye. nG, natural size.

Fritz Müller concluded from the repetition of the nauplius form in all orders of Crustaceans that the primitive form of the Crustacean must have been a nauplius, and that from it all the modern Crustaceans must have evolved phyletically by the addition of segments varying in number and differentiation. Now, however, it is doubted whether there ever were nauplioid types capable of reproduction. But even if the nauplii only represent what have been the larval forms from very early times, they are equally important in illustrating the relations between ontogeny and phylogeny; they at any rate represent the primitive pre-cambrian larval form from which all modern Crustaceans are derived. This is borne out not only by the facts to which we have already referred, but also by those Crustacean-groups which have diverged far from the usual Crustacean habit and type.

Fig. 111. Nauplius larva from the
winter egg of Leptodora hyalina; after
Sars.

Thus the sessile Cirrhipedes, with their mollusc-like shells, their soft, unsegmented bodies, degenerate heads, and their twelve vibratile food-wafting limbs, emerge from the egg as nauplius larvæ. But the remarkable parasites on the shore-crabs and the hermit-crab deviate much further from the type of the rest of the Crustaceans, for they hang like a sac or formless sausage-like soft mass to the abdomen of their host, growing into it by fine, pale, root-like threads, through which they suck up the blood of their hosts (Fig. 112, C. Sacc.). They possess neither head, nor thorax, nor abdomen, not even an indication of segmentation, no limbs of any kind, neither antennæ, nor mouth parts, nor swimming-legs. Nevertheless they are Crustaceans; indeed, we can say with certainty that they belong to the order of Cirrhipedes, for they leave the egg in the form of a nauplius larva (A), with 'horns' on their carapace which no other forms except themselves and the Cirrhipedes possess. That they are of the same stock as these is also proved by their further development, for the nauplius grows first, just as in the case of the Cirrhipedes proper, into a 'Cypris-like larva' (B), so called because it bears a certain resemblance to the Ostracods of the genus Cypris, and only from this point do their paths of development diverge. The Cypris-like larva of the true Cirrhipedes settles down somewhere, attached by its antennæ; it grows, and its body becomes that of the perfect Cirrhipede; but the Cypris-like larva of the Sacculinæ bores its way into the inside of a crab or hermit-crab, at the same time losing its limbs, segmentation, and its chitinous covering; and within the body of its host it is transformed into the sac-like organism we have already described. After a time it emerges again on the surface, and remains attached to the abdomen of its host (Fig. 112, C. Sacc.), drawing its nourishment from the blood which it sucks up by means of its numerous delicate roots (W, W).

Fig. 112. Development of the parasitic Crustacean Sacculina carcini, after Delage. A, Nauplius stage. Au, eye. I, II, III, the three pairs of appendages. B, Cypris-stage. VI-XI, the swimming appendages. C, mature animal (Sacc), attached to its host, the shore-crab (Carcinus mænus), with a feltwork of fine root-processes enveloping the crab's viscera. s, stalk. Sacc, body of the parasite. oe, aperture of the brood-cavity. Abd, abdomen of the crab with the anus (a).

From all this we may conclude that certain Cirrhipedes in times long past adopted a parasitic habit in the Cypris-larva stage, and that they gradually underwent adaptations to this mode of life, and that these went further and further, until the animal was transformed into the singular creature which we now see in the sexually mature form.

The same is the case with the numerous fish-parasites of the order Copepoda. They all leave the egg as nauplius larvæ, however greatly they may be modified later on by adaptation to a parasitic habit, and in them we can still observe, in the fully developed animals, a whole series of grades of transformation. Thus many genera, like Ergasilus, are distinguished from the free-swimming Copepods only by the modification of their jaws into piercing and sucking organs, and of a single pair of antennæ into hooks, by means of which they attach themselves to the fish on which they feed. In other genera the degeneration and modification go further; the antennæ, the eye, and the appendages degenerate more or less, and very remarkable attaching organs are sometimes developed, in the form of hooks or of knobbed pincers, or of actual suckers. In several types the degeneration and modification go so far that the segmentation of the body disappears, and the animal looks more like an intestinal worm than like a Crustacean (Lernæocera and others). In all these forms adapted to a parasitic mode of life it is always only the mature animal which has been transformed in this manner, for previously it has gone through a series of stages which are quite similar to those of the free-swimming Copepods, beginning with the nauplius, and ending with the so-called Cyclops stage, that is, a larval form which possesses antennæ, eyes, and swimming-legs similar to our freshwater Copepods of the genus Cyclops.

Here again we see in the ontogeny the repetition of a series of phyletic stages before the mature form is assumed. Why these stages should have persisted it is easy enough to understand, for how could an animal which emerged from the egg as a worm-shaped Lernæocera find a fresh fish which would serve it as host? Yet these parasites could not possibly go on preying upon the same fish generation after generation. To secure the existence of the species it was therefore indispensable that the faculty of swimming should be retained at least in the young stages; in other words, that the free-swimming ancestral stages should be preserved in the ontogeny. In all these cases it is therefore beyond doubt that the germinal history recapitulates a series of stages comparable to those of the racial history, although not quite unchanged but adapted to the modern conditions of life, for instance in having shorter antennæ, smaller eyes, and with four instead of the usual five swimming-legs. The search for a host does not seem to last long, for fishes are usually found in large numbers together, and thus the young parasitic Crustacean does not require to make a long journey before it finds a refuge.

It is noteworthy that the males of parasitic Crustaceans are not only much smaller than the females (Fig. 113), but that they are also much less modified, and resemble the ancestral free-swimming Copepods to a much greater degree. They usually possess small but well-developed swimming-legs, and by means of these they seek out the female, dying after fertilization is accomplished. They are thus not sessile parasites at all, and have therefore to go through the stages of the free-swimming Copepods much more completely than the females, whose task is to accumulate within themselves from the blood of the fish as much material as possible for the forming of the eggs, and to produce the largest possible number of these. These therefore greatly surpass the free-swimming Copepods in fertility, as is evidenced by the enormous egg-sacs they bear at the posterior end of the body (Fig. 113, ei).

Even among the higher Crustaceans, the so-called Malacostraca, the germinal history not infrequently exhibits more or less of the racial history in distinct recapitulation.

Fig. 113. The two
sexes of the parasitic
Crustacean Chondracanthus
gibbosus, enlarged
about six times; after
Claus. The main figure
is that of the female,
whose body bears quaint
blunt processes. At its
genital aperture (♂) a
dwarf male is situated.
F and F´, the two pairs
of appendages. ei, the
long egg-sacs, portions
of which have been cut
off in the figure.

It is true however, as we have already shown, that there are only a few of the higher Crustaceans which emerge from the egg in the form of a nauplius; in most of them this stage has been shunted backwards in the ontogeny, and most of the crabs and hermit-crabs leave the egg in a higher larval form, that of the so-called Zoæa (Fig. 114). This term is applied to a larva which already exhibits two main divisions of the body, a head and thorax portion (cephalothorax, Cph) and an abdomen (abd). The cephalothorax is frequently equipped with remarkable long spines (st), and it always bears from five to eight pairs of limbs, anteriorly the antennæ (I and II), then the mandibles (III), further back swimming-legs (IV, V), and behind these can be recognized the primordia of the other legs (VI-XIII), which will grow freely out later on. Large facetted and stalked eyes (Au) are borne on the head. This Zoæa form is not now found as a mature Crustacean form, so we cannot maintain with any confidence that it lived as a mature animal at an earlier period of the earth's history, but a second still more complex larval form of the higher Crustaceans is preserved for us in a group of marine Crustaceans, the Schizopods. These are Crustaceans which, though small, approach in external appearance our freshwater crayfish, only they have, instead of the ten walking-legs, biramose swimming-legs, by means of which they move freely in the water. The number of these branched legs is even greater than ten, there are sixteen of them ([Fig. 109] D, p. 164, VI-XIII). In the aquaria of the Zoological Station at Naples one may often see these dainty little creatures swimming about in large companies. Here they are of interest to us chiefly because their structure occurs in the ontogeny of the highest Crustaceans, the Decapods; that is, the phyletic stage represented by the Schizopods appears as an ontogenetic stage, just before the final metamorphosis of the larva to the perfect animal. This is the case in most of the marine Decapods, in those forms which do not go through the whole course of their development within the egg, but emerge as Zoæa larvæ, or even, as in Peneus potimirim, as nauplii. In the last-named species ([Fig. 109]) the ontogeny contains at least three stages which must have lived, perhaps not as mature forms, but as primitive larval forms, for unthinkable ages—the stage of the nauplus ([Fig. 109] A), that of the Zoæa ([Fig. 109] B and C), and that of the Schizopod ([Fig. 109], D); from this last the fully developed Decapod Crustacean arises ([Fig. 109], E).

We are, therefore, justified in saying that here the racial evolution is recapitulated in the individual development, although condensed and shortened in proportion as more numerous stages of the phyletic development are gone through within the egg, for there the different stages can succeed each other more rapidly and directly than in a metamorphosis of the free-swimming larvæ, since these must procure their own material for their further growth and their metamorphosis, while the yolk of the egg supplies a store of material which is sufficient for the production of a whole series of successive stages.

Fig. 114. Zoæa-larva of a Crab,
after R. Hertwig. I-V, the already
functional anterior appendages—antennæ,
mandibles, and swimming-legs.
VI-XIII, rudiments of the
posterior appendages of the cephalothorax
(Cph). Abd, the abdomen.
st, spine of the carapace. Au, eye.
H, heart.

For this reason it inevitably resulted that the sharply defined characters of the phyletic stages were more and more lost as soon as they were transferred from larval stages to stages in embryogenesis. For, in the first place, these sharply defined characters, such as the spines of the Zoæa larva, or the swimming bristles of the 'oars,' or the shape of thorax or abdomen characteristic of certain species, are adapted to a free life, and would be valueless in an embryonic stage; and secondly, in the transference of the free larval stages to embryonic development the greatest possible condensation and abbreviation of the stages must have been striven for, which could only come about by a continual mutual adaptation of the embryonic parts to one another, involving the suppression of everything superfluous. Otherwise the transference of the free stages to the embryogenesis would have brought no advantage, but rather a most prejudicial protracting of the development.

We must not, therefore, expect to find the stages of the phylogeny occurring unaltered in every ontogeny in the way we have found the nauplius, Zoæa, or Mysis stages in the larval development of the Decapods. I have noticed already that in the water-fleas (Daphnidæ) and other Crustaceans without metamorphosis the nauplius stage is still passed through, but within the egg, and as an embryonic stage, and this is quite true, but nevertheless it would hardly do to liberate a nauplius like this from its shell and place it in the water, for the influence of the water upon the delicate embryonic cells of its body would soon cause it to swell, and would destroy it utterly. And, even apart from this, it has no hard and resistant chitinous covering, no fully-developed appendages, but only the stump-like blunt beginnings of these without swimming-bristles and without muscles capable of function, so that it could not even move. Nevertheless it is a nauplius with all its typical distinctive characters, only it is not a perfect nauplius capable of life, but rather a 'schema' of one, which must be retained in the embryogenesis that it may give rise to the later stages.

Shall we therefore say that the statement that phylogeny repeats itself in ontogeny is false, that the nauplius stage within the embryo is not a true nauplius at all? That would be pushing precision beyond reasonable limits, and would obscure our insight into the causal connexion between phylogeny and ontogeny, which, as we have seen, undoubtedly exists.

A few years after the appearance of Fritz Müller's work Für Darwin, Haeckel elaborated Müller's idea, and applied it in a much more comprehensive manner. He formulated it under the name of 'the fundamental biogenetic law,' and then he used this 'law' to deduce from the ontogeny of animals, and more particularly of Man, the paths of evolution along which our modern species have passed in the course of the earth's history. In doing so the greatest caution was necessary, since ontogeny is not an actual unaltered recapitulation of the phylogeny, but an 'abridged' and in most cases—in my own belief, in all cases—a greatly modified recapitulation. Therefore we cannot simply accept each ontogenetic stage as an ancestral stage, but must take into consideration all the facts supplied to us by other departments of biological inquiry which afford help in the decision of such questions, especially those brought to light by comparative morphology and by the whole range of comparative embryology.

Haeckel was quite well aware of this difficulty, and repeatedly emphasized it by laying stress on the fact that a 'blurring' of the phyletic stages of development had arisen through the abridgement of the phylogeny in the ontogeny, and a 'falsification' of it through the secondary adaptation of individual ontogenetic stages to new conditions of life. He therefore distinguished between 'Palingenesis,' that is, simple though abridged repetition of the ancestral history, and 'Cœnogenesis,' that is, modification of the racial history by later adaptation of a few or many stages to new conditions of life. As an example of cœnogenetic modification, I may cite the pupæ of butterflies. Since these can neither feed nor move from one spot, they can at no time have been mature forms, and cannot, therefore, represent independent ancestors of our modern Lepidoptera; they have originated through the constantly increasing difference between the structure of the caterpillar and that of the moth or butterfly. Originally, that is, among the oldest flying insects, the mature animal could be gradually prepared within the larva as it grew, so that finally nothing was necessary but a single moult to set free the wings, which had in the meantime been growing underneath the skin, and to allow the perfect insect to emerge, complete in all its parts. This is the case even now with the grasshoppers and crickets. In these forms the larval mode of life differs very little, if at all, from that of the perfect insect, and the main difference between the two is the absence of wings in the larva. But when the perfect insect adapted itself to conditions of life quite different from the larval conditions, as was the case with the nectar-sucking bees and butterflies adapted entirely for flight, while the larvæ were still adapted exclusively to an abundant diet of leaves and other parts of plants, and to a very inactive life upon plants, the two stages of development ultimately diverged so widely in structure that the transition from one to the other could no longer be made at a single moulting, and a period of rest had to be interpolated, in order that the transformation of the body could take place. In this way arose the stage of the resting and fasting pupa, a 'cœnogenetic' modification of the last larval stage, not a recapitulation of an ancestral form, but a stage which has been interpolated, or better, has 'interpolated itself' into the ontogeny on account of the widely different adaptations of the early and the final stages.

This is a perfectly clear idea, and Haeckel's distinction between palingenesis and cœnogenesis is undoubtedly justified.

But it is quite a different matter to be able to decide whether a particular stage or organ has arisen palingenetically or cœnogenetically with the same certainty as in the case of the insect-pupa, or even with any degree of probability, and we must admit that in very many cases, perhaps even in most cases, it is impossible. This is so chiefly because pure palingenesis is hardly likely to occur now; the ancestral stages were bound to be modified in any case if they were to be compressed into the ever-shortening ontogeny of later descendants, and particularly so if they were to be shunted back into embryogenesis. In the latter case they would not only be materially shortened, and, as I have already shown, modified by the mutual adaptations of the different developing parts, but time-displacements of embryonic parts and organs would be necessary, as has been very clearly proved by the excellent recent investigations, which we owe in particular to Oppel, Mehnert, and Keibel. A shunting forward or backward of the individual organs takes place—conditioned apparently by the decreasing or increasing importance of the organ in the finished state; for in the course of the phylogeny everything may vary, and not only may a new, somewhat modified, and often more complex stage be added on at the end of the ontogeny, but each one of the preceding stages may vary independently, whenever this is required by a change in its relations to the other stages or organs. Adaptation is effected at every stage and for every part by the process of selection, for all parts of the same rank are ceaselessly struggling with one another, from the lowest vital units, the biophors, up to the highest, the persons. If we reflect that, in the course of the phylogeny of every series of species, a number of organs always become superfluous and begin to disappear in consequence, we can understand what great changes must take place gradually as such a series of phyletic stages is compressed into the ontogeny, for all organs which are no longer used are gradually shifted further and further back in the ontogeny till ultimately they disappear from it altogether. But, while the primary constituents of these 'vestiges' play their part in ontogeny for a shorter and shorter time, new acquisitions are being more and more highly developed, and thus, in the course of the phylogeny, numerous time-displacements of the parts and organs in ontogeny must result, so that ultimately it is impossible to compare a particular stage in the embryogenesis of a species with a particular ancestral form. Only the stages of individual organs can be thus compared and parallelized.

But we must not on that account 'empty out the child with the bath,' and conclude that there is no such thing as a 'biogenetic law' or recapitulation of the phylogeny in the ontogeny. Not only is there such a recapitulation, but—as F. Müller and Haeckel have already said—ontogeny is nothing but a recapitulation of the phylogeny, only with innumerable subtractions and interpolations, additions and displacements of the organ-stages both in time and place. It would be a great mistake to conclude from the fact of these manifold alterations that the whole proposition of the recapitulation of the phylogeny in the ontogeny is erroneous, or at least valueless. If its only use were to enable us to read the racial history of a species out of its germinal history, it is intelligible enough that we might be led to give it up in despair, but I think that the main thing is to get some insight into the history of the ontogeny, and there can be no doubt that this can have been built up on no other foundation than upon the racial history. What is new could only have arisen from what was already in existence, and everything in ontogeny, not only the palingenetic stages which still represent in some measure the facies of fully-formed ancestral stages, but also the cœnogenetic stages, like the pupa-stage we have already discussed, have arisen historically, nothing de novo, but all in connexion with what was already present. But what was first present was in all cases the stages of the ancestral forms.

It is undoubtedly of the greatest value to be able to penetrate more and more deeply into embryonic development, and to discover more precisely the changes that have taken place throughout its course in the originally existing material of ancestral forms. But it must not be forgotten that, all transformations notwithstanding, so much of the racial history is still very plainly indicated in the germinal history, that this must always remain for us a most important source from which to draw conclusions in regard to the phyletic development of any animal group. I admit that these conclusions have sometimes been drawn with too great confidence, but even if we cannot regard as well founded Haeckel's view that in the ontogeny of Man there are fourteen different ancestral stages recognizable, a protist stage, a gastræa stage, a prochordate, an acranial, a cyclostome, a fish-stage, and so on, we must recognize that the unicellular stage of ontogeny, with which even now the development of every human being begins, undoubtedly repeats the facies of an ancestor, although greatly altered; for we must be descended from unicellular organisms. The essential part of this ancestral stage is thus preserved in the ontogeny, and only what is special and in some measure due to chance, that is, to adaptation to special conditions of existence, has been modified.

It has been supposed that the proposition that phylogeny is recapitulated in the ontogeny is disproved, because the ontogenetic stage must always contain within it the primordia of the later stages which have been added since the corresponding phylogenetic stage. It is certain that the egg-cell or the sperm-cell of Man contains, though in a form not recognizable by us, all the determinants of the perfect human body, but this neither affects its nature as a cell nor its particular form as ovum or spermatozoon. It is essentials that are important in this comparison, not accessories. Neither can I agree with Hensen's argument when he says that the 'recapitulation-idea' is erroneous, because the actual course of ontogeny is the 'best and only possible one,' which, apart from previous history altogether, must of necessity be followed. Certainly the actual course is the best, and under the given circumstances the only possible one, but that does not exclude recapitulation, on the contrary it implies it, for ontogeny could at no time have arisen from a tabula rasa, but only from what was historically existent.

I do not propose to examine each of Haeckel's ancestral stages in Man's pedigree, or to estimate the degree of probability with which they may be deduced from the ontogeny; but that Man's ancestry does, in a general way, include such a series of phyletic stages may be admitted, even if we grant that many of these stages are now no longer represented in the ontogeny as stages of the developing organism as a whole, but only by stages of individual organs or group of organs. Thus it may be disputed whether there is still a fish-stage in human development, but it cannot be disputed that the rudiments of 'gill-arches' and 'gill-clefts,' which are peculiar to one stage of human ontogeny, give us every ground for concluding that we possessed fish-like ancestors.

As we now know that the history of a given mode of embryogenesis has involved numerous time-displacements of the organ-rudiments, we must attach all the more weight to the developmental history of the individual parts and characters, in which the phylogeny can often be read more clearly than in the stages of the organism as a whole, and we can probably find out important laws in this way.

As far back as 1873 Würtemberger investigated the fossil ammonites with special reference to this point. He was concerned even more at that time with finding proofs of the theory of descent in general, and this was the first case in which any one succeeded in demonstrating phyletic transformation-series of species, deposited one above the other in a corresponding series of geological strata, and connected by transition forms lying between these. In studying this interesting material, of which many examples were at his disposal, Würtemberger proved that the variations which had taken place in the spirally coiled shell in the course of ages appeared first on the last whorl, and then subsequently extended to the one before this, and thence to the still younger whorls of the shell. Meanwhile the last whorl not infrequently exhibited another new character. Thus, for instance, protuberances on the shell were shifted in the course of the phylogeny from the last convolution to the second last, and later to the third last, and so on, while at the same time the last convolution showed the protuberance changed into spines. In other words, the new phyletic acquirements first appeared in the mature animal (in the last-formed whorl or chamber of the shell), but were subsequently shifted back in the ontogeny to younger stages in proportion as new transformations of the mature animal appeared. Thus there was, so to speak, a retraction of the phyletic acquisitions of the mature animal deeper and deeper into the germinal history of the species.

About the same time—in the seventies—I obtained similar results from living species when I was attempting to work out the ontogeny of the markings on the external skin of the caterpillars of certain butterflies, and I should like to submit a short account of these.

In one of the early lectures we discussed the protective and defensive colours of caterpillars in general, and those of caterpillars of the Sphingidæ in particular. I showed that those naked caterpillars which live on plants among the grass, or on the grass itself, are often not only green, like fresh grass-stalks, or yellowish-grey, like dry ones, but all the larger forms also exhibit light, usually white, longitudinal lines, which, by mimicking the sharp light reflections on the grass-stems, heighten the protective resemblance.

We also spoke of the light transverse stripes, often marked with pink or lilac-blue, of many of the large green caterpillars which live on trees and bushes, and whose likeness to the leaves is heightened by this imitation of the lateral veining of a leaf; and finally we mentioned the warning coloration indicative of unpleasant or nauseous taste, among which must be classed not only vivid contrasts of colour, but also specially conspicuous elements of colour such as light ring-spots upon a dark ground. These different colour schemes which protect the caterpillars from their enemies are usually only to be found in the adolescent caterpillar, not in the very small one which has just emerged from the egg, and the development of the markings in the individual life clearly shows that the phylogeny of the markings is more or less obviously contained in the ontogeny.

There are three different schemes of marking which occur in the caterpillars of hawk-moths or Sphingidæ—longitudinal striping, obliquely transverse striping, and spots. Longitudinal striping pure and unmixed is now found only in a few species, for instance in the caterpillar of the Macroglossa stellatarum (Fig. 115), in which a white longitudinal line, beginning at the tip of the tail, runs up each side of the body to the head as a 'sub-dorsal stripe' (sbd). These, with other two similar stripes, effectively secure the fairly large caterpillar from discovery when it is among grass and herbs.

Fig. 115. Caterpillar of the Humming bird
Hawk-moth, Macroglossa stellatarum. sbd, the sub-dorsal
line.

Transverse striping occurs as the sole mode of marking in species which live on bushes and trees whose leaves have strong lateral veins, such as willows, poplars, oaks, privet, syringa, and so on, and these markings associated with the leaf-green of their colouring protect them most effectively from discovery.

The third scheme of marking, namely by spots, occurs in various forms in species of the genera Deilephila and Chærocampa, and it varies in its biological significance; in many species the spots serve as a warning colour, by making the caterpillar conspicuous and easily seen from a distance (Deilephila galii, [Fig. 117]); in others they imitate the eyes of a larger animal, and have a 'terrifying' effect, as we have already said (Fig. 4); in still other and rarer cases they heighten the resemblance of the caterpillar to its food-plant by mimicking parts of it, as, for instance, the red berries of the buckthorn (Deilephila hippophaës, Fig. 8, r).

Fig. 3 (repeated). Full-grown caterpillar of the
Eyed Hawk-moth, Smerinthus ocellatus. sb, the sub-dorsal
stripe.

Thus all three modes of marking possess a biological value, and protect the soft and easily wounded animal in some way, and, in the case of at least two of them, it is clear that they must have arisen at the very end of the caterpillar's development, since they can only be effective as the animal is approaching full size, and would be valueless in the very young caterpillar. The transverse striping only makes the caterpillar like a leaf when the stripes bear about the same relation to each other as those on the leaf, and eye-spots can only scare away lizards and birds when they are of a certain size. Only longitudinal striping is effective as a protection in the case of young caterpillars, supposing, that is, that they live in or on the grass (Fig. 116).

Fig. 4 (repeated). Full-grown caterpillar of the Elephant Hawk-moth, Chærocampa elpenor, in its 'terrifying attitude.'

Fig. 8 (repeated). Caterpillars of the Buckthorn Hawk-moth, Deilephila hippophaës. A, Stage III. B, Stage V. r, annular spots.

Let us consider the ontogeny of these different forms of markings, beginning with the eye-spots. It appears that these develop from a sub-dorsal stripe, which appears in the young caterpillar in the second stage of its life, and from it, in the course of the further development, two pairs of large eye-spots are formed. Even in the young caterpillar, scarcely one centimetre in length (Fig. 116), it can be observed that the fine, white sub-dorsal line takes a slight curve upwards on the fourth and fifth segments (C), and on the lower edge of these curves a black line is laid down (D). This is then continued to the upper side (E), and encloses the piece of the sub-dorsal stripe (F and G), and thus there arises a white-centred, black-framed spot which only requires to grow and to differentiate a blackish shadow-centre, the pupil (G), to give the impression of a large eye. This occurs as the caterpillar goes on growing, and after the fourth moult or ecdysis the eyes have already some effect, as the animal is six centimetres in length, but they become even more perfect in the fifth and last stage. During this development of the eye-spot the sub-dorsal stripe disappears completely from the greater part of the caterpillar, persisting only on the first three segments (Fig. 116, B-F).

Fig. 116. Development of the eye-spots in the caterpillar of the Elephant Hawk-moth (Chærocampa elpenor). A, Stage I, still without marking, simply green. B, Stage II, with sub-dorsal stripe (sbd). C, sub-dorsal line somewhat later, with the first hint of the eye-spot (Au) on segments 4 and 5. D, eye-spots in Stage III of the caterpillar, somewhat further developed than in E, the third stage. F, Stage IV. G, the anterior eye-spot at the same stage.

When we consider that this stripe in the little caterpillar a centimetre long, which lives on the large leaves of the vine, or on the obliquely ribbed willow-herb (Epilobium hirsutum), is quite without protective value, its occurrence at that stage can only be regarded as a phyletic reminiscence due to the fact that the ancestors of these species of Chærocampa possessed longitudinal stripes in the adult state, probably because at that time they lived on plants among the grass, and that, later, when the species changed their habitat to plants with broad leaves which had arisen in the meantime, eye-spots were developed in addition to the green or brown protective colouring which they retained. Thus the modern development of these spots mirrors their phyletic evolution very faithfully; on the two segments there were formed, from pieces of the sub-dorsal line, first white spots ringed round with black, then unmistakeable eyes with pupils (C, D, G). This transformation can only have begun in the fairly well-grown caterpillar, because it was only of any use to it; but later on it was shunted further back in the ontogeny, from the sixth and fifth to the fourth and third caterpillar stage, not in its complete development, but in more and more incipient form; and nowadays the first traces of eyes, as we have already seen, are visible in the course of the second stage. The marking of the more remote ancestors, the longitudinal striping, is now lost in proportion as the eye-spots develop, perhaps because the former would take away from the full effect of the latter. The longitudinal stripes are still quite plainly visible on the first three segments, but these segments are drawn in and are scarcely noticeable when the caterpillar assumes a defiant attitude (Fig. 4).

In the case of marking with ring-spots, which is found especially in species of the genus Deilephila, the ontogeny discloses that it has developed phyletically from the sub-dorsal stripe; in the young stage of this caterpillar also, the sole marking is longitudinal striping; in Deilephila zygophylli, from the steppes of Southern Russia, this persists apparently through all the stages, but in the others it disappears almost completely in the later stages, but only on the segments on which the spot-marking has developed from it. This happens in a manner similar to that in which the eye-spot in Chærocampa arises, a piece of the white sub-dorsal stripe is enclosed above and below by a semicircle of black, and later these semicircles unite, and cut off the portion of the sub-dorsal line, and form a black spot with a light centre within which a red spot frequently appears (Fig. 117, A).

Fig. 117. Caterpillar of the Bed-straw Hawk-moth
(Deilephila galii). A, Stage IV, sub-dorsal
stripe still distinct, the annular spots are still
incompletely enclosed in it. B, fully-formed
caterpillar without trace of a sub-dorsal stripe,
but with ten annular spots.

In most species these ring-spots occur on many segments (10-12) (Fig. 117, B), and in cases where they are of importance in making the caterpillar conspicuous and easily seen they sometimes form a double row. But we know one species, Deilephila hippophaës, in which only a single ring-spot exists, and it is a large brick-red spot on the second last segment, mimicking the red berry of the buckthorn (Fig. 8, A and B, r). But individuals also occur in which there are, on the five or six segments in front, smaller ring-spots which become less distinct the further forward they are, and in most caterpillars it is possible, on careful examination, to recognize little red dots on the faded sub-dorsal stripes of these segments (Fig. 8, B). We might be disposed to think, on this account, that the ancestors of D. hippophaës bore rings on all the segments, and that these had gradually become vestigial on the majority of them, because they had lost their earlier biological importance, and now, by adaptation to the buckthorn, could only be of use on the second last. But when we take the ontogeny also into account we find in the young caterpillar only a simple sub-dorsal line, upon which, in the third stage, the red spot of the tail-horn segment appears (Fig. 8, A).

No spots ever occur on the other segments at this stage; they only appear in the last stage, but as they may be entirely wanting, they must have arisen as the result of internal laws of correlation, that is, they must be recapitulations of the hindmost spots which arose in the phylogeny through natural selection. We may conclude this, at least, if we believe in the truth of the fundamental proposition of the biogenetic law, and admit that there is in the ontogeny some more or less distinct recapitulation of the phylogeny.

Fig. 118. Two stages in the life-history of the Spurge Hawk-moth (Deilephila euphorbiæ). A, first stage, the caterpillar dark blackish-green, without marking. B, second stage, the row of spots is distinctly connected by a light streak, the vestige of the sub-dorsal stripe.

This proposition may be recognized as true in the case of Deilephila also, if we compare the different species with one another as regards their ontogeny. We find here too that not only the sub-dorsal, that is, the phyletically oldest marking of the Sphingid caterpillars, occurs everywhere in the young stages, but also that it is being shunted back to younger and younger stages, in proportion to the degree of the development of the spot-marking reached in the full-grown caterpillar. Thus, for instance, in the caterpillar of Deilephila euphorbiæ the highest form of spot-marking is reached, and in this species the sub-dorsal line is no longer the sole marking element at any stage. Leaving out of the question the absolutely unmarked little caterpillar which emerges from the egg (Fig. 118, A), there appears at once in the second stage a series of ring-spots connected by a fine white sub-dorsal line (Fig. 118, B). In the following stage, the third, this sub-dorsal line disappears without leaving a trace, and there remains only the spot-marking, which is subsequently duplicated.

Let us compare with this the ontogeny of the bed-straw hawk-moth, Deilephila galii (Fig. 117). The full-grown caterpillar possesses only a single row of ring-spots (B), and accordingly the young stages of the caterpillar up to the fourth show a distinct sub-dorsal line (A), although spots are seen upon it. A still earlier phyletic stage of development is illustrated by Deilephila livornica, in which the ring-spots are all connected by the sub-dorsal line.

It can thus hardly be doubted that the biogenetic law is guiding us aright when we conclude from a comparison of the ontogeny of the different species of Deilephila, that the oldest ancestors of the genus possessed only the longitudinal stripes, and that from these small pieces were cut off as ring-spots, and that these were gradually perfected and ultimately duplicated, while at the same time the original marking, the longitudinal stripe, was shunted back further and further in the young stages, until it finally disappeared altogether.

Let us now refer for a moment to the third form of marking in the caterpillars of the Sphingidæ—transverse striping. This has not arisen out of the sub-dorsal line, but quite independently and at a later date. This is proved with great certainty by the ontogeny of species of the genus Smerinthus. The full-grown, and usually also the young caterpillars, of these species have quite regularly the seven broad oblique stripes which run in the direction of the tail-horn at equal intervals on the lateral surfaces of the body ([Fig. 3]). They are absent only from the three anterior segments, and upon these a part of the older marking, the sub-dorsal stripe, has persisted. But we find this fully developed in the youngest stages of other species. In Smerinthus populi, the little caterpillar, which has no markings at all when it leaves the egg, very soon shows the white sub-dorsal line, and simultaneously with it the seven transverse stripes, which cut obliquely through it; in the older caterpillars the sub-dorsal then disappears (Fig. 119).

When I was investigating these matters at the beginning of the seventies I did not succeed in procuring eggs of the species of the genus Sphinx, which likewise almost all exhibit the oblique striping in their full-grown stages. But from what I knew of the ontogeny of Smerinthus species I was able to predict that, among the young stages of Sphinx, there must be some with sub-dorsal lines. This was confirmed later, for Poulton found in Sphinx convolvuli that in the first stage there are no oblique stripes, but only the sub-dorsal stripe, while in Sphinx ligustri both kinds of marking were present at the same time.

Fig. 119. Caterpillar of Smerinthus populi, the Poplar Hawk-moth, at the end of the first stage, showing both the complete sub-dorsal stripe and the oblique stripes.

From all these facts, which I have summarized as briefly as possible, we see that the older phyletic characters are gradually crowded by the newer into ever-younger stages in the ontogeny, until ultimately they disappear altogether. We have now to ask to what this phenomenon is due; is it a simple crowding out of the old and less advantageous by the new and better characters as a result of natural selection, or is there some other factor at work? It is clear in regard to these forms of marking that they can have been developed at first only in the almost full-grown larva by natural selection, because they are of use only there, and that, at the same time, the old marking must have been set aside through the influence of the same factor, in as far as it prejudiced the effect of the new adaptation. This seems to be indicated by the persistence of the sub-dorsal line on those segments which are drawn in when Chærocampa assumes a terrifying attitude, or which do not bear oblique stripes in the leaf-like caterpillars, e.g. the three anterior segments in the species of Sphinx and Smerinthus. When newly acquired schemes of marking like the eye-spots of Chærocampa are transmitted from the last stage to the stage before, this can be explained by following the same train of thought, for the caterpillar is already of sufficient size to be able to inspire terror with its eyes; but in still younger stages the spots would not be likely to have that effect, and yet they occur in quite small animals (20 mm.). More obvious still is the uselessness of the oblique striping in the young stages of the Sphinx and Smerinthus caterpillars, for in the earliest stages of life the caterpillars are much too small to look like a leaf, and the oblique stripes stand much closer together than the lateral ribs of any leaf. Moreover, the little green caterpillars require no further protection when they sit on the under side of a leaf; they might then very easily be mistaken in toto for a leaf-rib. Thus it is certainly not natural selection which effects the shunting back of the new characters. Nor can this be caused by the fact that the new character can only be developed gradually and in several stages, for the oblique striping at any rate arises in the ontogeny all at once. There must therefore be some mechanical factor in development to which is due the fact that characters acquired in the later stages are gradually transferred to the younger stages. But this shifting backwards can be checked by the agency of natural selection as soon as it becomes disadvantageous for the stage concerned.

It is in this way that I explain the fact that the majority of the caterpillars of the Sphingidæ are absolutely without markings when they emerge from the egg. Thus, for instance, the caterpillars of Chærocampa (Fig. 116, A), of Macroglossa (Fig. 115), and of Deilephila (Fig. 118, A), as well as those of the Smerinthus species, are at first without stripe or mark of any kind; they are of a pale green colour, almost transparent, and very difficult to recognize when they sit upon a leaf. How very greatly the different stages can be independently adapted to the different conditions of their life, when that is necessary for the preservation of the species, is shown in the most striking manner by many species. Thus the little green caterpillar of Aglia tau, when it leaves the egg, bears five remarkable reddish rod-like thorns, which in form and colour resemble the bud-scales of the young beech-buds among which they live, and which disappear later on; the full-grown caterpillar shows nothing of these, but is leaf-green, marked with oblique stripes. Even if the use of these reddish thorns be other than I have indicated, we have in any case to deal with a special adaptation of one, and that the first caterpillar-stage, and what can happen at this stage is possible also at every other. Nor is it only animals which undergo metamorphosis that can exhibit independent phyletic variation at every stage, but those also with direct development, and indeed, in the case of these, we may assume adaptation of this kind at almost every stage in the history of the organs, as we have already seen, because the great abridgement of the phylogeny into the ontogeny necessitates a very precise mutual adaptation of the organ-rudiments and of the diverse rates of development.

We have thus been led by the facts discussed—and numerous others from other groups in the animal kingdom might be ranked along with them—to two main propositions, which express the relation of phylogeny to ontogeny. The first and fundamental proposition is the one already formulated. The ontogeny arises from the phylogeny by a condensation of its stages, which may be varied, shortened, thrown out, or compressed by the interpolation of new stages. The second proposition refers to individual parts, and may run as follows: As each stage can undergo new adaptations by itself, so can every part, every organ; such new adaptations very often show a tendency to be transferred to the immediately antecedent stage in ontogeny.

It is not my intention to formulate the laws of ontogeny just now, otherwise many others might be added to these, such as that of the regular transference of characters acquired at one end of a segmented animal to the other segments: I must confine myself here to bringing the two main propositions into harmony with the principles of our theory of heredity.

How phylogeny is condensed in ontogeny can be understood readily enough in a general way, although we cannot profess to have any insight into the detailed processes. The continuity of the germ-plasm brings about inheritance, in that it is continually handing over to the germ-plasm of the next generation the determinant-complex of the preceding one. Every new adaptation at any stage whatever depends on the variation of particular determinants within the germ-plasm, and this in its turn depends on germinal selection, that is, on the struggle of the different determinant-variants among themselves, and on the variation in a definite direction which arises from this, as we have already shown. A new kind of determinant can never arise of itself, but always only from already existing determinants, and through variation of these. But as spontaneous variation never causes all the homologous determinants of a germ-plasm to vary in quite the same way, but only a majority of them, there always remains a minority of the old determinants, which may, under certain circumstances, predominate again, as is proved by the aberrations in Vanessa species due to cold, and by many other kinds of reversion.

But it is not this variation which leads to the prolongation of ontogeny, and the repetition of the phyletic stages within it. In this case it is rather that a new character takes the place of an old one, not that it is added to it. A black spot may arise instead of a red one, but not first a black spot and then a red one. Of course we still know far too little in regard to the intimate succession of events in the stages of ontogeny to be able to say definitely that, in such apparently simple transformations, the older stage does not, in every ontogeny, precede the more recent one as a preparation for it, though it may be only for a brief and transient period.

It is certain, however, that variations such as the addition of a new stage in ontogeny are undergone, and that this implies the occurrence of something really quite new. Therefore such a new stage can arise only from the germ-plasm, by the duplication, and in part variation, of the determinants of the preceding stage. If, for instance, the body of a Crustacean be lengthened by a segment, this must be due to a process of this kind, and in such a case it is intelligible enough that the new segment can be formed in the ontogeny only after the development of the older preceding one, for its determinants come from that, and are from the beginning so arranged that they are only liberated to activity by the formation of the preceding segment.

Now, if in the course of the phylogeny numerous new segments were added to the body of the Crustacean, the ontogeny would be materially prolonged, and condensation would become necessary in the interests of species-preservation. To bring this condensation about, whole series of segments which were added successively in the phylogeny succeeded each other with gradually increasing rapidity in the ontogeny, until finally they appeared simultaneously: the determinants of the segments n, n + 1, n + 2, ... n + x varied in regard to their liberating stimuli, and were roused to activity no longer successively, but simultaneously, in the cell complexes controlled by them. We have thus recapitulation, but with abridgement and compression, of the phyletic stages in the ontogeny. Thus in the nauplius of Leptodora we see the rudiments of five of the pairs of legs of the subsequent thorax ([Fig. 111], IV-VIII), and in the Zoæa larva the rudiments of six thoracic legs may be seen behind the already developed swimming-leg ([Fig. 114], VI-XIII).

But in the course of the phylogeny a segment may also become superfluous, and we know that it then degenerates and is ultimately eliminated altogether. Thus in a parasitic Isopod, which lives within other Crustaceans, a segment of the thorax is wanting in the relatively well-developed larva, and in the Caprellidæ among the Amphipod Crustaceans the whole abdomen of from six to seven segments has degenerated to a narrow, rudimentary structure. In such cases the gradual degeneration of the relative determinants has preceded step for step the degeneration of the part itself, and when this is complete the ontogeny shows nothing of what was previously present, and so we may speak of a 'falsification' of the phylogeny. But that the complete disappearance of the determinants only comes about with extreme slowness, so that whole geological periods are sometimes not enough for its accomplishment, we have already learnt from our study of rudimentary organs, instances of which can be demonstrated in every higher animal, bearing witness to the presence of the relevant organs or structures in the ancestors of the species.

We can infer with certainty, from the observational data at our disposal, that the disappearance of useless parts is regulated by definite laws; but it is too soon to attempt to formulate these laws, or even to trace them back to their mechanical causes. As we have already said, a much more comprehensive collection of facts, and above all one which has been made on a definite plan, is a necessary preliminary condition to this. But so much at least we may gather from the facts before us, that the degeneration of an organ begins at the final stage, and is transferred gradually backwards into the embryogenesis. Thus the two fingers of birds which have disappeared since Cretaceous times are still indicated in every bird-embryo, though they subsequently degenerate. In various mammals 'pre-lacteal tooth-germs' have been demonstrated in the jaws of embryos, which show us that not only did ancestors exist whose dentition was the modern 'milk-teeth,' but that still more remote ancestors possessed another set of teeth, which was crowded out by the 'milk-teeth'; thus the teeth of the ancestors of the modern right whale (Balæna mysticetus) are only represented in the embryo of to-day in the form of dental pits. And, as we saw already, the Os centrale so characteristic of the wrist of lower vertebrates only appears in Man at a very early embryonic stage, and disappears again as such in the further course of the embryogenesis.

We may perhaps give a preliminary statement of this law as follows: It is impossible that any part or organ should be removed suddenly from the ontogeny without bringing the whole into disorder, and the least serious disturbance of the course of development will undoubtedly be caused if the final stage of the part in question become rudimentary first. Only after this has happened, and the neighbouring parts have adapted themselves to the disappearance, can this extend to the stages immediately preceding it, so that these too degenerate, and allow the surrounding parts to adapt themselves. The further back into the ontogeny the disappearance extends the greater will be the number of other structures affected in some way or other by the degeneration, and these must not all be brought suddenly into new conditions, else the whole course of development would suffer. Thus at first only those determinants may disappear—and can disappear according to the laws of germinal selection—which control the final form of the useless organ, then those just preceding them, which controlled, let us say, its size, and thus more and more of the previously active determinants disappear, and hand in hand with this disappearance there is variation of all the parts correlated with the dwindling condition of the organ, so that their own development and that of the animal as a whole suffers no injury. If it were otherwise, if when a part became useless its collective determinants were all to disappear at the same time, the whole ontogeny would totter, in fact it would be much as if a man who wished to remove the breadth of a window from a house standing on pillars were to begin by taking away the foundation pillar.

It is, of course, to be understood that these processes go on so exceedingly slowly that personal selection takes a share in them, at least at the beginning. Later on, the further degeneration of a useless organ or rudiment has no effect on the individual's power of life, and therefore depends solely upon the struggle of the parts within the germ-plasm (germinal selection).

If we could see the determinants, and recognize directly their arrangement in the germ-plasm and their importance in ontogeny, we should doubtless understand many of the phenomena of ontogeny and their relation to phylogeny which must otherwise remain a riddle, or demand accessory hypotheses for their interpretation. Several years ago Emery rightly pointed out that the phenomena of the variation of homologous parts might be inferred by reasoning from the germ-plasm theory. If one hand has six fingers instead of five, it not infrequently happens that the other also exhibits a superfluity of fingers, and sometimes the foot does so too. The phyletic modification of the limbs in the Ungulates has taken place with striking uniformity in the fore and hind extremities; no animal has ever been one-hoofed in front and two-hoofed behind. Although I might suggest that this primarily depends on adaptation to different conditions of the ground, and that the Artiodactyls were evolved in relation to the soft marshy soil of the forest, and the Perissodactyls for the steppes, it cannot be denied that germinal conditions may have co-operated in bringing about this uniformity of the direction of variation, especially as the whole structure of the fore- and hind-limbs exhibits such marked similarity. Emery is inclined to refer this to 'germ-plasmic correlations,' and we have assumed from the very first that the different determinants and groups of determinants do indeed stand in definite and close relations to one another. But it seems to me premature to say anything more precise and definite than that in the meantime. I should like, however, to say that determinants or groups of determinants which had in old ancestral germ-plasms to give rise to a series of quite similar structures by multiplication during the ontogeny, and therefore only needed to be present singly in the germ-plasm, would, in later descendants, have to shift their multiplication back into the germ-plasm itself, if necessity required that the homologous parts which they controlled should become different from each other. Then the previously single group of determinants in the germ-plasm would have to become multiple. But as new determinants can only arise from those which already exist, these new ones must have had their place beside the old, and would therefore probably be exposed to any intra-germinal causes of variation in common with them—that is to say, they will tend to vary even later in a similar manner. For instance, we might think of the segments of primitive Annelids, which in form and contents are for the most part alike, as arising from one germ-rudiment, from which, when, in the higher Annelids, the various regions of the body had to take a different form, several primary constituents of the germ-plasm separated themselves off; and in a similar way the much higher and more complex differentiation of the somatic segments in the Crustaceans must have been brought about. Thus we understand how the determinant groups of the germ-plasm multiplied according to the need for increasing differentiation, but remained in intimate relation, which exposed them in some measure to a common fate, that is, to common modifying influences, and in many cases determined them to similar variation.

But we cannot see directly into the germ-plasm, and are therefore thrown back on the inductions we can make from the facts presented to us by the phenomena of visible living organisms. As yet the material for such inductions is scanty, because it has been got together haphazard, and not collected on a definite plan. I therefore refrain for the present from attempting any further elaboration of my germ-plasm theory. It is only when an abundance of observation material, collected according to a definite plan, lies at our disposal that anything more in regard to the intimate structure of the germ-plasm, or the mutual influences and relations of its determinants and its modification in the course of phylogeny can be deduced with any certainty. Meanwhile, we must content ourselves with having, through the hypothesis of determinants, made intelligible at least the one fundamental fact, how it is possible that in the course of the phylogeny single parts and single stages can be thrown out or interpolated, or even only caused to vary, without giving rise to variation in all the rest of the parts and stages of the animal. A theory of epigenesis cannot do this, for, if no representative particles were contained in the germ-plasm, then every variation of it would affect the whole course of development and every part of the organism, and variations of individual parts arising from the germ would be impossible.

---