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.