LECTURE XVII
THE GERM-PLASM THEORY
Conception of the 'id' deduced from the process of fertilization—Hereditary substance, 'idioplasm' and 'germ-plasm'—'Idants'—Evolution or Epigenesis—Herbert Spencer's uniform germinal substance—Determinants—Illustrations: Lycæna agestis—The leaf-butterflies—Insect metamorphosis, limbs of segmented animals—Heterotopia—The ultimate living units or biophors—Number of determinants—Stridulating organ of the grasshopper.
In proceeding to expound the theory of heredity which has shaped itself in my mind in the course of my own scientific development, I should like to begin by pointing out that the hereditary substance of the germ-cell of an animal or of a plant contains not only the primary constituents (Anlagen) of a single individual of the species, but rather those of several, often even of many individuals. That this is so can be proved in several ways.
I start from what I hold to be the proved proposition, that the chromatin substance of the nucleus is the hereditary substance. We have seen that this is present in the germ-cells of every species in the form of a definite number of chromosomes, and that in germ-cells destined for fertilization, that is, in sex-cells, this number is first reduced to half, the reduction being effected, as is now proved in regard to a whole series of animals, by the two last cell-divisions, the so-called maturation divisions.
We know that the full number is only reached again through amphimixis, by which process the half number of chromosomes in the male and female germ-cells are united in a single cell, the 'fertilized ovum,' and in a single nucleus, the so-called segmentation nucleus. Thus the hereditary substance of the child is formed half from the paternal, half from the maternal hereditary substance, and we have seen that this remains so during the whole development of the child, since, at every succeeding cell-division each of the paternal and each of the maternal chromosomes doubles by dividing, and the resulting halves are distributed between the two daughter-nuclei.
Now if the complete hereditary substance of a germ-cell before the reducing divisions contains potentially all the primary constituents of the body, which it does as a matter of course, then it follows that after the reduction each germ-cell must either contain only half the primary constituents of the parents or all the primary constituents must be contained in the half number of chromosomes. The latter seems to me the only possible assumption, as I shall immediately proceed to show, and this is as much as to say that the primary constituents of at least two complete individuals must be contained in the chromosomes of the segmentation nucleus.
That this conclusion is correct is obvious from the fact that a whole, that is, a perfect individual with all its parts, develops from the ovum, and not a defective one. For suppose that each mature germ-cell contained only half the primary constituents of the body, it would be impossible that these halves should always exactly complete themselves to form a whole embryo when they are brought together in fertilization, after having been halved by mere chance during the preceding reducing division; it would be much more likely to happen that they did not complete themselves, and that their union would therefore result in an individual with certain parts wanting. If, for instance, in the sperm-cell only the anterior half of the body was potentially present, and this united with an ovum which likewise contained only the primary constituents of the anterior half, the embryo resulting from their union would lack the posterior half of the body, and so on. Of course so rough a division of the primary constituents is not to be thought of, but however fine we can imagine the halving of the mass of primary constituents to be, there would never be any guarantee that the two cells uniting in amphimixis would complete the mass of primary constituents again; indeed, the chance that the two exactly complementary halves of the mass would meet would rather become less the finer and more complex one imagines the halving by reducing divisions to be. A perfect embryo with all its parts would rarely arise, but now one group of parts, now another would be wanting, while another group might be developed double, or at least would be doubly present in the primary constituents.
But in addition to this the facts of inheritance show us that the resemblance to mother and father may express itself simultaneously in all the parts, or at least in the same parts of the child, as may be seen with especial clearness among plant-hybrids, and thus the conclusion is inevitable that even in the half number of chromosomes all the primary constituents of the whole body are present.
Let us go a generation further. If the species possess four chromosomes the child will have in its cells two maternal chromosomes (A) and two paternal chromosomes (B); what form will this proportion take in the germ-cells produced by the child? The maturation division can effect the reduction to two chromosomes in different ways; there may, for instance, be two paternal chromosomes (B) left in the one, and two maternal chromosomes (A) in the other daughter-cell, or one paternal (B) and one maternal (A) in the one, and a similar combination in the other cell. Let us follow the latter case further. A sperm-cell which contained the combination A and B might meet in amphimixis with an egg-cell of different origin also containing a similar combination of chromosomes, let us say a chromosome C from the mother, and a chromosome D from the father. We should then have in the segmentation nucleus of the fertilized ovum four different chromosomes, each of which contained the hereditary substance of one grandparent; we should have the four chromosomes, A, B, C, D, as the hereditary substance of the grandchild.
But since, as we have seen, the halved hereditary substance still contains the whole mass of primary constituents, each one of these chromosomes must contain the collective primary constituents of the whole body of the relevant grandparent[18]. The hereditary substance in the fertilized ovum thus consists of several complexes of primary constituents (chromosomes) each of which (an 'id') comprises within itself all the primary constituents of a complete individual.
[18] When I say the 'collective' primary constituents of the whole body of the grandparent this is not expressing it quite precisely, for, as we shall see later, each individual must arise from the co-operation of different chromosomes of different origin, not merely from one of the chromosomes contained in its germ-plasm. In the example given above, the body of each grandparent cannot have arisen only from a single chromosome, which was transmitted to his grandchild, but from the co-operation of this chromosome with three others, which have distributed themselves along other genealogical paths. But this does not affect the above chain of reasoning, for here it is not a question of whether all the primary constituents of the grandparent are present in the child—that can never be the case—but whether the primary constituents transmitted by him represent the whole body of an individual.
It can be made clear in yet another way that, as a consequence of sexual reproduction, the germ-plasm of each species must be composed of several 'ids,' individually different. Let us assume that there was as yet no amphimixis, and that we could look on at its introduction into the organic world; the hereditary substance of the beings which had previously lived and multiplied by division would consist of more or less numerous chromosomes similar to each other, so that, for instance, each individual would contain sixteen identical 'ids.' But if amphimixis were now to take place for the first time, in the same manner as it does to-day—that is, after the reduction of the number of the ids to half—in the first amphimixis eight paternal ids would unite with eight maternal ids to form the germ-plasm of the new individual, as is indicated in [Fig. 87] by a circle of spheres, of which ten are white and ten black as a sign of their difference. We may think of the figure as representing the 'equatorial plate' of a nuclear spindle with its ids arranged in a circle. Now, if two organisms of this generation, with two kinds of ids, unite in amphimixis after previous reduction of the ids, we have figure B, in which the paternal ids (pJ) are seen to the left of the line and the maternal ids (mJ) to the right, while each semicircle is in its turn made up of two kinds of ids, those of the grandparents (p2J and m2J, p2J1 and m2J1). The figures C and D show the two following generations, in which the number of identical ids is each time reduced to half, because eight strange ids are again mingled with them; in C only two ids are still identical, and in D all the ids are individually different, because they have come from different ancestors of the same species. Of course this would only be the case if inbreeding were excluded, because through it the ids of the same forefathers from two or more sides would meet; but prolonged inbreeding is a rare exception in free nature.
Fig. 87. Diagram to illustrate the operation of amphimixis on the composition of the germ-plasm out of diverse ancestral plasms or 'ids.' A-D, the ids of the germ-plasm of four successive generations: A, consisting of only two kinds of ids; B, of four; C, of eight; D, of sixteen kinds. pJ and mJ, paternal and maternal ids. p2J, grandpaternal; p3J, great-grandpaternal; p4J, great-great-grandpaternal ids. The marks in the ids themselves indicate their individually distinct characters.
I shall now call the hereditary substance of a cell its 'idioplasm,' after Nägeli's example, although he sought it in the cell-substance, not in the nucleus, and had a different theoretical conception of its mode of action. It was he, however, who conceived and established the idea of the idioplasm as the bearer of the primary constituents, an Anlagensubstanz, determining the whole structure of the organism in contrast to the general nutritive protoplasm. Every cell contains idioplasm, since every cell-nucleus contains chromatin, but I call the idioplasm of the germ-cells germ-plasm, or the primary-constituent-substance of the whole organism, and the complexes of primary constituents necessary to the production of a complete individual—whose presence we have just shown to be theoretically necessary—I call ids. In many cases these 'ids' might be synonymous with chromosomes, at least in all the cases in which the chromosomes are simple, that is, are not composed of several similarly formed structures. Thus in the salt-water Crustacean, Artemia salina, which possesses 168 minute granular chromosomes, each of these chromosomes must be regarded as an id, for each can in certain circumstances be thrown out from the ovum by the reducing division, or it can be brought into the most various combinations with other chromosomes by fertilization. Each of them must therefore consist of perfect germ-plasm in the sense that all the parts of an individual are virtually contained in it; each is a biological unity, an id. But when we see in many animals larger band-shaped or rod-shaped 'chromosomes,' and when these are composed of a series of granules, as they are, for instance, in the often mentioned Ascaris megalocephala, each of these granules is to be regarded as an id. In point of fact, we find, instead of the two or four large rod-shaped chromosomes of Ascaris megalocephala, a larger number of smaller spherical chromosomes in other species of Ascaris.
Compound chromosomes consisting of several ids, such as all rod or band-like elements of the nuclear substance probably are, I designate 'idants.' That they are composed of several individual ids is not always clearly apparent because of the smallness of the object, and even in larger ones this may only be seen in certain stages. Thus we have in [Fig. 88], A and B, two 'mother-sperm-cells' of the salamander; A at an earlier stage, in which the individual ids are not visible; B at a later stage, in which the band has split, and the rosary-like structure has become at once apparent. It is not possible, then, to see at once whether each chromosome corresponds to one or to several ids. A more exact investigation of the processes of reducing division has shown that there are chromosomes of simple spherical form, that is, composed of several ids whose 'plurivalence' cannot be directly recognized, but can only be inferred from their further development; there are bivalent chromosomes of double value and quadrivalent chromosomes of fourfold value, which we have to think of as made up of two or four ids. It would lead us too far to go into this more precisely, nor does it fall within the scope and intention of these lectures to inquire into these intimate and still disputed details.
The germ-plasm of every species of plant or animal is thus composed of a larger or smaller number of ids or primary constituents of an individual, and it is through the co-operation of these that the individual which develops from the ovum is determined.
Fig. 88. Sperm-mother-cells (spermatocytes) of the salamander. A, cross-section of the cell in the aster-stage; the chromosomes (chr) or idants do not reveal that they are compounded out of many ids, which are, however, quite distinctly seen in B (Jd), where the chromosomes or idants (chr) are already longitudinally split. zk, cell-substance. csp, centrosome. c, centrosome in division. After Hermann and Drüner.
We have further to inquire what conception we can form of the constitution of an id and of its mode of operation. I have already spoken of 'primary constituents' (Anlagen) of which the germ-plasm consists, but what right have we to think of the parts of an animal as already contained in the germ in any form whatever? Is it not equally possible that the germ consists of parts, none of which bear any definite relation in advance to the parts of the finished animal? Might not the germ-cell, along with its nucleus, undergo transformations and regular changes which would successively give rise to new conditions, namely, the different stages of development, until finally the complete animal was attained?
We stand here before an old problem, before the two opposed interpretations—the theory of 'Evolution' and the theory of 'Epigenesis,' which were first ranged against each other long ago, and which are a cause of strife even now, although in somewhat different guise.
The theory of 'Evolution' is especially associated with the name of Bonnet, who elaborated it in detail in the eighteenth century. It maintains that the development of the ovum to the perfect animal is not really a new creation, but only an unfolding of invisible small parts, which were already present in the ovum. It assumes that the parts of the perfect organism are already preformed in the ovum, and on this account it is called the 'Preformation Theory.' Bonnet often speaks of the preformation of the perfect animal in the germ as a 'miniature model,' although his conception of 'evolution' was not really so crude as has been often alleged. He expressly emphasized that this miniature model was not exactly like the perfect animal, but consisted of 'elementary parts' only, which he thought of as a net whose meshes were filled up during development and by means of nutrition with an infinite number of other parts. But after all, his conceptions, and those of his time generally, were very far removed from the biological thinking of our own day, as may perhaps be most readily understood when I mention that he regarded death and decay as an 'involution,' as a folding back, so to speak, by means of which all the parts gained though nutrition were removed again, so that the net of the miniature model shrank together to the invisible minuteness that it had in the ovum. So it remained, he fancied, till it was reawakened at the resurrection, using the term in the religious sense! He afterwards dropped this fancy, because the objection was made to it that human beings who had lost a leg or an arm in this life would necessarily be maimed at the resurrection!
In Bonnet's time the facts of development were quite unknown, and not even the stages of the development of the chick from the egg had been observed. When this was afterwards done the prevalent theory of 'evolution' necessarily collapsed, for men saw with their own eyes that a miniature model of the chick did not gradually grow into visibility and ultimately into the young chick, but that first of all parts showed themselves in the egg which bore no resemblance at all to the chick, that these first rudiments were then altered, and that through continual new formations and transformations the chick finally appeared. Upon this K. von Wolff based his theory of 'Epigenesis,' or development through new formations and transformations. He maintained that the doctrine of 'Evolutio' was false; that there is no miniature model invisibly contained within the egg; but that from the simple egg-substance there arises, through the agency of the formative powers inherent in it, a long series of stages of development, of which each succeeding one is more complex than the one before, until ultimately the perfect animal is reached.
This certainly marked considerable progress, for it meant the beginning of a science of embryology, that is, the science of the form-development of the animal or plant from the ovum. The result was not so important in its theoretical aspect, for though the knowledge had been gained that the young animal goes through a long series of different stages, it had not been discovered how nature works this wonder and causes an animal of complex structure to arise from the apparently simple substance of the ovum. A solution of the difficulty was found by attributing to the ovum a formative power, afterwards called by Blumenbach the nisus formativus, which possessed the capacity of developing a complex animal from the simple 'slime,' or, as we should say, the simple protoplasm.
If we contrast the strictly theoretical part of the two theories, we find that Bonnet regarded the ovum as something only apparently simple, but in reality almost as complex as the animal which developed from it, and that he thought of the latter, not as being formed anew, but as being unfolded or evolved. That is to say, he thought that rudiments present from the outset in the ovum gradually revealed themselves and became visible. Wolff, on the other hand, regarded the ovum as being what it seemed, something quite simple, out of which only the nisus formativus could, by a series of transformations and new formations, build up a new organism of the relevant species.
Wolff's Epigenesis routed Bonnet's theory so completely from the field that, until quite recently, epigenesis was regarded as the only scientifically justifiable theory, and a return to the 'evolutionist' position would have been looked upon as a retrograde step, as a reversion to a period of fancy which had been happily passed. I myself have been repeatedly told, with regard to my own 'evolutionistic' theory, that the correctness of epigenesis was indisputably established, that is, was a fact, verifiable at any time by actual observation!
But what are the facts? Surely only that there is a succession of numerous developmental stages, which we know very precisely in the case of a great many animals, and that the miniature model which Bonnet assumed to be in the egg does not exist. Both these facts are now no longer called in question. But that does not furnish us with a theory of development, for theory is not the observation of phenomena or of a series of phenomena, it is the interpretation of them. Epigenesis, as formulated first by Aristotle and again by Harvey, Wolff, and Blumenbach, certainly offered an interpretation of development, not, however, by referring only to what was observable, but by going far beyond it; on the one hand taking the appearance of a homogeneous germ-substance for reality, and, on the other, assuming a special power, which caused a heterogeneous organism to arise from a homogeneous germ.
We cannot now accept either of these assumptions, for we know that the germ-substance is not homogeneous, and indeed is not merely a substance but a living cell of complex structure; and we no longer believe in a special vital force, and therefore not in a special 'power of development,' which could only be a modification of the former. We are thus as little able to accept the old epigenesis as the old evolution, and we must establish a theory of Development and Heredity on a new basis.
What this basis must be is in a general way beyond doubt. Since it is the endeavour of the whole of modern biology to interpret life more and more through the interactions of the physical and chemical forces bound up with matter, development, too, comes within this aim, for development is an expression of life. We seek to understand the mechanism of life, and, as a part of that, the mechanism of development and of heredity which is closely associated with it.
If we wished to attack the problem of heredity at its roots we should first of all have to try to understand the process of life itself as a series of physico-chemical sequences. Perhaps this will be achieved up to a certain point in the future, but if we were to wait for this we should in the meantime have to abandon all attempts at a theoretical interpretation of the phenomena of development and heredity, and might indeed have to postpone them to the Greek Kalends. That would be as though, in the practice and theory of medicine, all investigation into and speculation regarding disease had to wait until the normal, healthy processes of life were thoroughly understood. In that case we should now know nothing of bacteria diseases and the hundred other acquisitions of pathological science: physiology too would have remained far behind its present level if it had lacked the fruitful influence of experience in cases of disease, and the ideas and theories, true and false, which have been based thereon. In the same way we require a theory of development and heredity if we are to penetrate deeper into these phenomena, and must have it in spite of the fact that we are still very far from having a complete causal knowledge of the processes of life. For the raw material of observation, which is to some extent fortuitous, will never bring us any further on; observation must be guided by an idea, and thus directed towards a particular goal.
It is, however, quite possible to leave aside for the present all attempts at an explanation of life, and simply to take the elements of life for granted, and on this basis to build up a theory of heredity. We have already taken a step in that direction by establishing that the whole substance of the fertilized ovum does not take part in heredity in the same degree, but that only a small part, the chromatin of the nucleus, is to be looked upon as the bearer of the hereditary qualities, and by deducing, further, that this chromatin is made up of a varying number of small but still visible units, the ids, each of which virtually represents the whole organism, or, as I have already expressed it, each of which contains within itself, as primary constituents, all the parts of a perfect animal.
It was these 'primary constituents' which led us to the digression in regard to Bonnet's theory of 'Evolutio' and Wolff's 'Epigenesis.'
Let us now inquire what must be the constitution of such a chromatin globule, an id, so that, shut up within the nucleus of a living reproductive cell, it can direct the development of a new organism which resembles its parent. Two fundamental assumptions present themselves, and these can be related to every conception of a 'germ-plasm,' even independently of the assumption of ids. Either we may think of the id as made up of similar or of different kinds of parts, none of which has any constant relation to the parts of the perfect animal, or we think of it as composed of a mass of different kinds of parts, each of which bears a relation to a particular part of the perfect animal, and so to some extent represents its 'primary constituents' (Anlagen), although there may be no resemblance between these 'primary constituents' and the finished parts. The assumption of a germ-plasm composed of similar parts, which has been made, for instance, by Herbert Spencer, may be called the modern form of epigenesis, while the other assumption is the modern form of the 'evolution' theory. As the former theory can no longer call to its aid a 'formative power' as a Deus ex machina, it can only explain development as induced by the influence of external conditions—temperature, air, water, gravity, position of parts—upon the chemical components of the germ-plasm, which are everywhere uniformly mingled; and it makes no difference whether this uniform germ-plasm is thought of as composed of many different kinds of parts, as long as those parts are mingled uniformly to make the germ-plasm and bear no relation to definite parts of the developing animal. Oscar Hertwig has recently outlined such a theory. Although I cannot expound it here I must say at least so much with regard to it, and to all other theories of development founded on a similar basis, that they could not be accepted even if they were able to offer a workable explanation of the development of the individual, and for this reason, that ontogeny is not an isolated phenomenon which can be interpreted without reference to the whole evolution of the living world, for it is most intimately associated with this, being indeed a piece of it, having, as we shall see, arisen from it, and, furthermore, preparing for its continued progress. Ontogeny must be explained in harmony with phylogeny and on the same principles. The assumption of a germ-plasm without primary constituents, or of a completely homogeneous germ-plasm, as Herbert Spencer maintained, is irreconcilable with this, for, as will be seen, it contradicts certain facts of inheritance and variation. Therefore all theories founded on this assumption must be rejected.
There is another and, I believe, weighty consideration which forbids us to assume a germ-substance without primary constituents. I shall return to this later, but in the meantime I wish to build up more completely my own 'germ-plasm' theory.
I assume that the germ-plasm consists of a large number of different living parts, each of which stands in a definite relation to particular cells or kinds of cells in the organism to be developed, that is, they are 'primary constituents' in the sense that their co-operation in the production of a particular part of the organism is indispensable, the part being determined both as to its existence and its nature by the predestined particles of the germ-plasm. I therefore call these last Determinants (Bestimmungsstücke), and the parts of the complete organism which they determine Determinates, or hereditary parts.
It is easy to show on what basis this assumption rests; the phenomena of inheritance taken in conjunction with those of variation seem to me to compel us to it. We know that all the parts of an organism are variable, and that in one individual the same part may be larger, in another smaller. Not all variations are transmissible, but many of them, and some very minute ones, are. Thus, for instance, in many human families there occurs a small pit, hardly as large as the head of a pin, in the skin of the ear, whose transmission I have observed from the grandmother to the son and to several grandchildren. In such a case there must be a minute something in the germ-plasm, not present in that of other human beings, which causes the origin, in the course of development, of this little abnormality in the skin.
There are human families in which individuals occur repeatedly, and through several generations, who have a white lock of hair, in a particular spot, on an otherwise dark-haired head. This cannot be referred to external influences, it must depend on a difference in the germ, on one, too, which does not affect the whole body, not even all the hairs of the body, but only those of a particular spot on the surface of the head. It is a matter of indifference whether the white colouring of the hair-tuft is produced by an abnormal constitution of the matrix of the hair, or by other histological elements of the skin, as of the blood-vessels or nerves. It can only depend ultimately on a divergently constituted part of the germ-plasm, which can only affect this one spot on the head, and alter it, if it is itself different from what is usual. On this account I call it the determinant of the relevant skin-spot and hair-group. In Man such minute local variations are usually lost after a number of generations, but in animals there are innumerable phenomena which prove to us that single minute deviations can become permanent. Thus there lives in Central Europe a brown 'blue butterfly,' Lycæna agestis, which has a little black spot in the middle of its wing. The same species also occurs in Scotland, but there, instead of the black spot, it has a milk-white one, and so-called 'eye-spots' on the under surface of the wing have also lost their black centres. The species has thus varied transmissibly, but only in regard to these particular spots on the wing. A slight variation must therefore have taken place in the germ-plasm which only affects these few parts of the body, or, to express it otherwise, the germ-plasms of the ancestral species and of the variety can only be distinguished by a difference which determines exclusively the scale colour of these spots. The two germ-plasms differ, I should say, only as regards the determinants of these wing-scales.
We know from the artificial selection to which Man has subjected and still subjects his domesticated animals and useful plants, that any spots and parts of the body which he chooses can be hereditarily altered, if the desired variations which present themselves are always selected for breeding, and that this does not necessarily cause variation in other parts of the body. When, for instance, in the case cited by Darwin, the comb of a Spanish cock which had previously hung downwards was made to stand upright because a prize had been offered for this character, or when a certain breed of hens was 'furnished with beards,' the results were permanent variations affecting only the parts on which the fancier's attention had been fixed. In the same way, when the tail feathers of the Japanese cock are lengthened to three feet the rest of the plumage does not alter, still less any other part of the body. Of course there are numerous 'correlated' variations, and in very many cases the breeder causes a second or third character, on which he had not fixed his attention, to vary in addition to the one he was aiming at. But such concomitant variations are not necessary or inevitable in all cases; and indeed we need not refer them all to a true correlation of the parts, but may suppose that they depend not infrequently on the faultiness of our power of observation, which is not sufficiently keen to control several parts of the body at one time, and to notice minimal variations in parts on which we have not specially fixed our attention.
Fig. 13. Kallima paralecta, from India;
showing the right under surface in the
resting pose. K, head. Lt, palps. B, limbs.
V, fore wing. H, hind wing. St, 'tail' of the
latter, representing the stalk of the leaf.
gl1 and gl2, transparent spots, Aufl, remains
of 'eye-spots.' Sch, a 'mould'-spot.
So much, at least, is certain, that in all these cases of the artificial alteration of individual characters the germ-plasm is in some way changed, but always in such a way that it differs from that of the ancestral form through such variations alone, and the effect of these is that only the altered parts are influenced thereby, and not the whole organism. This again is but another way of saying that only the determinants of these parts have altered.
We can see from a thousand cases that exactly the same happens in a state of nature, that there, too, one part changes after another, until the highest possible degree of adaptation to the conditions has been attained. In the mimetic resemblance to leaves exhibited among butterflies this is most clearly seen, for here we are familiar with the model—the leaf—and we see how one species approximates to it in a general way only in the total colour, how others develop a brown stripe crossing the posterior wing obliquely, so that, to a certain extent, it resembles the midrib of a leaf, how in a third species this stripe is continued for some distance forward across the anterior wing, in a fourth it goes a little further, until, finally, in a fifth, it is continued on to the tip of the anterior wing. This may be seen, for instance, in the genus Anæa, which is rich in species. But even then a still further increase of the resemblance is possible, for, as is well known, there are not infrequently imitations of the lateral veins of the leaf as well, or dark spots which faithfully reproduce the mould-spot on a damp, decaying leaf, or colourless transparent spots which probably simulate dewdrops, and so on. All these are variations relating to individual and distinct groups of wing-scales, which have varied transmissibly and independently, that is, each of them has been produced by a variation in the germ-plasm, which brought about a change in this particular area of the body and in no other.
Let us for a moment assume the impossible, and suppose that we could look on at the evolution of such a leaf-butterfly; the beginning of the leaf-imitation might have its cause in the fact that an ancestral form of Kallima, which had previously lived in the meadows, exhibited on the part of some of its descendants a migration to the woods, and thus divided into two groups, with a different manner of life—a meadow form and a wood form. The latter adapted itself to sitting among leaves, and the midrib of a leaf developed on its wings. In a germ-plasm without 'primary constituents' this variation could only depend on a uniform variation of all the parts, for these parts are either alike among themselves, or at any rate have the same value for every part of the finished organism. But the germ-plasm of the new breed must somehow differ from that of the ancestral form, otherwise it could produce no new variety, but only the ancestral form over again. But how could an animal differing only in one minute part arise from a germ-plasm which has varied in all its parts, and how could such little steps of variation be repeated many times in the course of the phylogeny without the corresponding variations of the germ-plasm becoming so intense that not only the wing-markings but everything about the animal would be altered likewise? And yet these 'leaf-pictures' have not originated suddenly, but by many small steps, so that the germ-plasm must have varied in toto a hundred times in succession if there are no primary constituents.
In the Indian species, Kallima paralecta, there are no fewer than five well-marked varieties, the differences between which depend solely on the manner in which the leaf-picture on the wing is elaborated, for the upper surface of the wing is alike in all. Even a cursory observation of a collection of these butterflies shows that the lateral veins of the leaf-picture are quite different in number, distinctness, and length in the different individuals. On the right half of the wing there may be as many as six of them indicated ([Fig. 13]); and it can be observed that the three middle ones are the longest, most sharply defined, and darkest, while those lying near the tip and the base of the mimic leaf are shorter and often even shadowy. On the left side the second lateral vein in particular distinctly shows indentations indicative of the rings, inherited from the ancestral forms, which surrounded the still visible eye-spots (Aufl); the third lateral vein is quite indefinite and shadowy, but nevertheless it runs exactly parallel to the first two, and thus heightens the deceptive effect. We can thus distinguish older and more recent elements in the marking—a proof of the slow and successive origin of the picture.
This is not reconcilable with the conception of a germ-plasm without primary constituents, however complex a mixture it may otherwise be. A substance which had to undergo thousands upon thousands of variations, arising from each other according to law and in the strictest succession, in order that it might become a definite organism, predetermined as to all its thousands of parts down to the most minute, cannot vary over and over again in its whole constitution without the consequences showing themselves in numerous, or indeed in all, the parts of the body. Such variations in the germ-plasm would be comparable to many successive deviations of a ship from her course, which, although the single ones would only cause a minimal deviation from the true course, would, when summed up in a voyage of some length, land the vessel at quite another coast than the one intended. If each individual adaptation of the species depended on a variation of the whole germ-plasm the wood Kallima would soon retain no resemblance to its ancestral form, the meadow species; yet we are acquainted with species of Kallima which do not show the special resemblance to a leaf, but, for instance, still exhibit the perfectly developed eye-spot of the ancestral form, and so forth. It follows, therefore, that the origin of the leaf-picture has not greatly influenced the general character of the species; and the fact that the upper surface of the wings has remained the same in all the varieties is in itself enough to prove this.
Since, then, the resemblance to a leaf cannot have arisen without something in the germ-plasm varying, since the germ-plasm of a forest Kallima and a meadow Kallima must be different in something, and cannot be any more alike than the germ-plasm of a fantail-pigeon and a carrier, there must be 'primary constituents' in the germ-plasm, that is, vital units whose variation occasions the variation of definite parts of the organism, and of these alone.
Fig. 17. Caterpillar of Selenia tetralunaria on a twig of birch. K, head. F, feet. m, protuberances resembling dormant buds. Natural size.
It is on such considerations as these that my assumption, that the germ-plasm is composed of determinants, depends. There must be as many of these as there are regions in the fully-formed organism capable of independent and transmissible variation, including all the stages of development. Every part, for instance, of the butterfly's wing, which is capable of independent and transmissible variation, must, so I conclude, be represented in the germ-plasm by an element which is likewise variable, the determinant; but the same must be true of every independently and transmissibly variable spot of the caterpillar from which the butterfly developed. We know how markedly caterpillars are adapted in form and colour to their environment. Let us assume that the caterpillar of the butterfly which we chose as an example of wing-marking had the habit of feeding only by night and during the daytime of resting on the trunk of a tree, or, more precisely, in the crevices of the bark. It would then resemble the caterpillar of the moths of the genus Catocala or the Geometers (Geometridæ), and possess the colour of the bark of the tree in question; the determinants of the skin would thus have varied to correspond with this mode of life on the part of the caterpillar, so that the skin would appear grey or brown. But there cannot be only one determinant of the caterpillar skin in the germ-plasm, for the bark-like colour of, for instance, a Geometer caterpillar is not a uniform grey, but has darker spots at certain places and lighter whitish spots at others, such as are to be seen on the bark of the twig on which the caterpillar is wont to rest, or brown-red spots, like those on the cover-scales of the buds, or little warts and protuberances which exactly correspond to similar roughnesses on the twigs, to cracks in the bark, and so on. All these markings are constant, and are to be found in the same spot in every caterpillar of the species. A large number of regions of the caterpillar skin must therefore be independently determined by the germ-plasm; the germ-plasm must contain parts the variations of which bring about variations only of an independently variable region of the caterpillar skin. In other words, in the germ-plasm of the butterfly ovum there must not only be determinants for many regions of the butterfly's wing, but also for many regions of the caterpillar's skin.
This line of argument, of course, applies to all the bodily parts and organs of the butterfly and of the caterpillar, as well as to all the stages of development of the species as far as these parts are able to vary in such a way that the variation reappears in the following generation, that is to say, as far as it is transmissibly variable.
But all parts must be transmissibly variable which have exhibited independent variation in relation to their ancestors. When, for instance, the eggs of a butterfly (Vanessa levana) bear a deceptive resemblance to the flower-buds of the stinging-nettle on which the caterpillar lives, not only in form and colour, but in their pillar-like arrangement, we may conclude that these eggs have varied transmissibly from those of their ancestors, which had not acquired the habit of living on the stinging-nettle, in these three respects independently, that is, uninfluenced by any other variations the species may have undergone; and that, consequently, the germ-plasm must contain determinants for the egg-shell, egg-colouring, and so on. The manner of laying the eggs in the form of pillars depends on a modification of the egg-laying instinct, which must in its turn depend on the variations of certain nerve-centres, and we learn from this that there must be in the germ-plasm determinants for the individual centres of the nervous system.
It may, perhaps, be suggested that matters could be explained in a simpler way—that it is enough to assume the presence in the egg of determinants for all the parts of the caterpillar, and that those of the butterfly are only formed within the caterpillar.
This suggestion seems justifiable if we confine ourselves to superficial considerations. We read in every handbook of entomology that the wings only arise during the life of the caterpillar, and in a certain sense this is true, for the primary constituents or primordia of wings only develop into the fully formed wing during the larval period. But even if these primordia were only formed during the caterpillar-stage, what could they develop from? Only out of the material parts of the caterpillar, that is, from some of its living cells or cell-groups. The constitution of the wings would therefore be dependent on that of the cells of the caterpillar from which they arose, so that if these varied transmissibly through the variation of their determinants contained in the germ, the determinants of the butterfly which were just developing would vary with them; every transmissible variation of the caterpillar would necessarily cause a similar variation in the butterfly, and this does not happen. If any one hazarded the assumption that the determinants of the butterfly develop only in the caterpillar, but quite independently of its constitution, he would either be making an absurd statement, namely, that the characters of the butterfly were not transmissible at all, or he would be unconsciously admitting that the determinants of the butterfly were already contained in the parts of the caterpillar, and come direct from the germ-plasm.
That the characters of the butterfly do vary independently of those of the caterpillar I demonstrated many years ago, when we were still very far away from the idea of the germ-plasm or of determinants. I demonstrated then that the constancy of the markings of a species can be quite different in the two chief stages; that the caterpillar may be very variable, while the butterfly or the moth may be very constant in all its markings, or conversely. I called attention to the dimorphic caterpillars which are green or brown, and yet become the same moth (for instance, Deilephila elpenor and Sphinx convolvuli); I cited the case of the spurge hawk-moth (Deilephila euphorbiæ), whose dark but at the same time motley caterpillars occur in the Riviera at Nice as a local variety (Nicæa), and there wear quite a different dress—pale clay-yellow, with a double row of large conspicuous dark yellow eye-spots—while the moth does not differ from our variety in a single definite character, except in its larger size. At that time, too, I instituted experiments with the caterpillars of the smallest of our indigenous Vanessa species (Vanessa levana), of which the majority are black with black thorns, while a minority are yellowish-brown with yellow thorns; reared separately, both yielded the same butterfly, though in this case one would be inclined to suppose that there was some internal connexion between the colour of the caterpillar and that of the butterfly, since the butterfly also occurs in two colours. It was shown, however, that the colour of the butterfly had nothing to do with that of the caterpillar, for it is known to be dependent on the season, and is a seasonal dimorphism, 'while the two forms of caterpillar may occur side by side at all times of the year.'
Subsequently I made a similar experiment with the dimorphic caterpillars of the 'fire'-butterfly (Polyommatus phlæas), and it yielded the same result. The pure green caterpillars became the same butterflies as those marked with broad red longitudinal stripes, and in this case we can definitely describe both colours as protective, for the green form is adapted to the green under surface of the leaf, the red-striped to the green red-edged stalk of the lesser sorrel (Rumex acetosella).
There was really no necessity for special proofs that the caterpillar and butterfly vary transmissibly in complete independence of each other, for the facts of metamorphosis alone are enough to prove it. How would it have been possible otherwise that the jaws adapted for biting should, in the primitive insects, and in the locusts which are nearest to them, remain as a biting apparatus throughout life, while in the caterpillar they are modified during its pupal stage into the suctorial proboscis of the butterfly? The parts of insects, therefore, must be capable of transmissible variation in the stages of life independently of each other. Not only have the jaws of the leaf-eating caterpillars remained unaltered, while in the sexually mature animal they have been gradually modified into a very long and extremely complex suctorial apparatus, but when at a much later time this proboscis became superfluous in a species, because the butterfly or moth, from some cause or another, lost the habit of taking any nourishment at all, its degeneration exercised no effect on the jaws of the caterpillar, as we can observe in many hawk-moths, silk-moths and Geometridæ. How could such a degeneration become transmissible if the caterpillar's jaws, from which those of the adult are developed, remain the same? We are thus forced to assume that there is something in the latter which can vary from the germ, without the jaws themselves being altered thereby. This 'something' it is which I call 'determinants,' vital particles, which—however we may try to picture them—are indeed contained in the cells of the caterpillar's jaws, but are there inactive and do not influence the structure of these, while, on the other hand, it is their constitution which determines the form and structure of the suctorial proboscis of the butterfly down to the minutest details. It must be these alone which cause the suctorial proboscis to develop, and in some cases to degenerate again, without bringing about any change in the corresponding parts in the caterpillar.
Fig. 89. Anterior region of the larva
of a Midge (Corethra plumicornis). K, head.
Th, thorax. ui, inferior imaginal disks.
oi, superior imaginal disks. ui1, ui2, and
ui3, the primordia of the limbs. oi2
and oi3, the primordia of the wings and
'balancers.' g, brain. bg, chain of ventral
ganglia with nerves which enter the
imaginal disks. trb, tracheal vesicle.
Enlarged about 15 times.
This example seems to me to be preferable to that of the wings of insects in this respect, that there is no organ in the caterpillar with a specific function corresponding to the wing of the butterfly. Yet the two cases are exactly alike, and it would be a mistake to say that the first primordium of the wing within the caterpillar is not a part of the caterpillar at all. At first, certainly, it is only a group of cells on the skin, occurring at a particular spot on the dorsal surface of the second and third segments of the caterpillar, and doubtless arising from a single cell of the embryo, the 'primitive wing-cell,' which, however, has not as yet been demonstrated. But it is nevertheless an integral part of the caterpillar, which could neither be wanting, nor be larger or smaller, and so on; which, in short, does mean something for the caterpillar, although perhaps not more than any other of the skin-cells. For the butterfly, however, this area on the skin means the rudiment of the wing; for from it alone can there arise by multiplication the aggregate of cells which grows out into a hollow protuberance, enlarges by degrees into a disk, the imaginal disk, and eventually develops into the form of wing peculiar to the species. This imaginal disk is connected very early with nerves and with tracheæ, as may be beautifully seen especially in dipterous larvæ (Fig. 89, oi), and these become later the nerves and tracheæ of the wing, while thousands of peculiar scale-like hairs develop on the upper surface; in short, the rudiment becomes a perfect wing with its specific venation, and with the marking and colouring which is often so complicated in Lepidoptera. Almost every little spot and stripe of the latter is handed down with the most tenacious power of transmission from generation to generation, and each can at the same time be transmissibly varied; the same is true of the venation, which is so important systematically just because it is so strictly hereditary, yet it too can vary transmissibly, as can also the hooked bristles, the odoriferous apparatus, and, in short, the whole complex structure of the wing, with all its specific adaptations to the mode of flight, to the manner of life, and to the colour of the environment. How is it possible that all this can develop from a skin-cell? Is it the influence of position that effects it, and could any other cell of the caterpillar's skin do the same if it were placed in the same position? Could any neighbour-cell of the primitive wing-cell replace it if it were destroyed? It is hardly probable, and I think I can even prove that this is not so. The experiment of killing such a cell in the living animal has not yet been made; if it should succeed, we may venture to say in advance that none of the neighbouring skin-cells will be able to do its work and take its place in developing a wing; the wing in question will simply remain undeveloped. In the summer of 1897 I hatched a specimen of Vanessa antiope from the pupa, which, though otherwise normal and well-developed, lacked the left posterior wing altogether; no trace of it could be recognized. In this case, from some cause which could no longer be discovered, the first formative cell of the wing in the hypodermis, or its descendants, must have been destroyed, and no substitution of another took place, as the defect showed.
The young science of developmental mechanics attributes to the position of a cell in the midst of a group of cells a determining value as regards its further fate, and as far as the cells of the segmenting ovum are concerned this seems to be true in certain cases, but the assumption cannot be generally true except in a very subordinate sense. The formative cell of the wing does not become what it is because of its relative position in the organism. If this were so it could not happen that a wing should develop instead of a leg, as was observed in a Zygæna, nor could there be any of those deformities already referred to, to which the name 'Heterotopia' is applied, and which consist in the development of organs of definite normal structure, or at any rate of apparently normal structure in quite unusual places, e. g. an antenna on the coxa of a leg, or of a leg instead of an antenna (in Sirex), or instead of a wing. It is therefore not some influence from without that makes that particular skin-cell of the caterpillar the rudiment of the wing, but the reason lies within itself, in its own constitution. As the whole mass of determinants for the whole body and for all the stages of its development must be contained within the ovum and the sperm-cell, so the primitive cell of the butterfly's wing must contain all the determinants for the building up of this complicated part; and if the cell gets into a wrong position in the course of development because of some disturbance or other, a wing may develop from it in that position if the conditions are not too utterly divergent. These heterotopic phenomena afford a further proof of the existence of determinants, because they are quite unintelligible without the assumption of 'primary constituents' or Anlagen.
The hypothesis of determinants in the germ-plasm is so fundamental to my theory of development that I should like to adduce another case in its support and justification. The limbs of the jointed-footed animals, or Arthropods, originally arose as a pair on each segment of the body, and they were at first alike or very similar both in their function and in their form. We find illustration of this in the millipedes, and still more in the species of the interesting genus Peripatus, which resembles them externally, as well as in the swimming and creeping bristle-footed marine worms (Chætopods) belonging to the Annelid phylum. We can quite well picture to ourselves that the whole series of these appendages was represented in the germ-plasm by a single determinant or group of determinants, which only required to be multiplied in development. Without disputing whether this has really been the case in the primitive Arthropods or not, it is certain that it can no longer be the case in the germ-plasm of the Arthropods of to-day. In these each pair of appendages must be represented by a particular determinant. We must infer this from the fact that the several pairs of these appendages have varied transmissibly, independently of each other, for some are jaws, others swimming legs, or merely bearers of the gills or of the eggs; others are walking legs, digging legs, or jumping legs. In Crustaceans a forceps-like claw is often borne by the first of the otherwise similarly constructed appendages, or also by the second or the third, or there may be no forceps, and so on; in short, we see that each individual pair has adapted itself independently to the mode of life of its species. This could only have been possible if each was represented in the germ-plasm by an element, whose variations caused a variation only in that one pair of legs, and in no other.
It may perhaps be objected that the differences in the appendages may quite well have had their origin simply during the development of the animal, while the primary constituents were the same for all, so that a single determinant in the germ-plasm would suffice. But this could only be the case if the differences depended not on internal but on external causes, that is, if the same primary constituents gave rise to a set of appendages which became different because they were subject in the course of their development to different modifying influences. But this is not the case, at least not to the extent that this supposition would necessitate. Can it be supposed that, for instance, the jumping legs of the water-flea (Gammarus) are a necessary consequence of the somewhat divergent form of the segments from which they grow? A direct proof to the contrary may be found in 'Heterotopia,' for in the place where a posterior limb, modified for holding the eggs, normally occurs in the crab an ordinary walking leg may exceptionally develop (Fig. 90, Bethe), or an appendage resembling an antenna may take the place of an extirpated eye (Herbst). But if there were really only one determinant in the germ-plasm for all the appendages these would of necessity be all alike, apart from the larger or smaller differences which might be stamped upon them by growing from segments different in size and in nutrition. Such differences, however, are far from being sufficient to explain the great deviations seen among the appendages of most kinds of Crustaceans, and still less to explain their adaptation to quite different functions.
Fig. 90. The Common Shore-Crab (Carcinus mænas), seen from below, with the abdomen forced back. In place of the swimmeret, which ought to be borne by the fifth abdominal swimmeret, a walking leg has grown on the left side, and one which properly should belong to the right side (6). 1-5, thoracic limbs, ps1-4, swimmerets of the right side. s6, s7, posterior segments of the abdomen. After Bethe.
It need not be imagined that my argument can be controverted by saying that one appendage-determinant in the germ may split itself in the course of development into a series of different appendage-determinants. The question would then arise, How is it able to do so? And the answer can be no other than that the single first determinant had within it several different kinds of elements, which subsequently separated to determine in different ways the various appendages. But that is just another way of saying that this single determinant actually includes within itself several different determinants. For a determinant means nothing more than an element of the germ-substance by whose presence in the germ the specific development of a particular part of the body is conditioned. If we could remove the determinants of a particular appendage from the germ-plasm this appendage would not develop; if we could cause it to vary the appendage also would turn out differently.
In this general sense the determinants of the germ-plasm are not hypothetical, but actual; just as surely as if we had seen them with our eyes, and followed their development. Hypothesis begins when we attempt to make creatures of flesh and blood out of these mere symbols, and to say how they are constituted. But even here there are some things which may be maintained with certainty; for instance, that they are not miniature models, in Bonnet's sense, of the parts which they determine; and, further, that they are not lifeless material, mere substances, but living parts, vital units. If this were not so they would not remain as they are throughout the course of development, but would be displaced and destroyed by the metabolism, instead of dominating it as living matter alone can do—doubtless undergoing oxidation, but at the same time assimilating material from without, and thereby growing. There cannot be lifeless determinants; they must be living units capable of nutrition, growth, and multiplication by division.
And now we have arrived at the point at which a discussion of the organization of the living substance in general can best be interpolated.
The Viennese physiologist, Ernst Brücke, forty years ago promulgated the theory that living matter could not be a mere mixture of chemical molecules of any kind whatever; it must be 'organized,' that is, it must be composed of small, invisible, vital units. If, as we must certainly assume, the mechanical theory of life is correct, if there is no vital force in the sense of the 'Natur-Philosophie,' Brücke's pronouncement is undoubtedly true; for a fortuitous mixture of molecules could no more produce the phenomena of life than a single molecule could, because, as far as our experience goes, molecules do not live; they neither assimilate, nor grow, nor multiply. Life can therefore arise only through a particular combination of diverse molecules, and all living substance must consist of such definite groups of molecules. Shortly after Brücke, Herbert Spencer likewise assumed the reality of such vital 'units,' and the same assumption has been made in more recent times by Wiesner, De Vries, and myself. In the meantime we can say nothing more definite about the composition of these bearers of life, or 'biophors,' as I call them, than that albumen-molecules, water, salts, and some other substances play the chief part in their composition. This has been found out by analysis of dead protoplasm; but in what form these substances are contained in the biophors, and how they affect each other in order to produce the phenomena of life by going through a ceaseless cycle of disruptions and reconstructions, is still entirely hidden from us.
We have, however, nothing to do with that here; we content ourselves with recognizing in the biophors the characteristics of life, and picturing to ourselves that all living substance, cell-substance, and nuclear substance, muscle-, nerve-, and gland-substance, in all their diverse forms, consist of biophors, though, of course, of the most varied composition. There must be innumerable kinds of biophors in all the diverse parts of the millions of forms of life which now live upon the earth; but all must be constructed on a certain fundamental plan, which conditions their marvellous capacity for life; all possess the fundamental characters of life—dissimilation, assimilation, growth, and multiplication by division. We must also ascribe to them in some degree the power of movement and sensibility.
As to their size, we can only say that they are far below the limits of visibility, and that even the minutest granules which we can barely perceive by means of our most powerful microscopes cannot be small individual biophors, but must be aggregates of these. On the other hand, the biophors must be larger than any chemical molecule, because they themselves consist of a group of molecules, among which are some of complex composition, and therefore of relatively considerable size.
It may now be asked whether the determinants, whose existence we have already inferred, are not identical with these 'biophors' or smallest living particles; but that is not the case, at least not generally. We called determinants those parts of the germ-substance which determine a 'hereditary character' of the body; that is, whose presence in the germ determines that a particular part of the body, whether it consists of a group of cells, a single cell, or a part of a cell, shall develop in a specific manner, and whose variations cause the variations of these particular parts alone.
Again, it may be asked how large and how numerous such 'hereditary parts' may be, whether they correspond to every distinct part of a cell, or to every cell of the body, or only to the larger cell groups. Obviously the areas which are individually determined from the germ must differ in size, according as we have to do with an organism which is small or large, simple or more complex. Unicellular organisms, such as Infusorians, probably possess special determinants for a number of cell-organs and cell-parts, although we cannot directly observe the independent and transmissible variation of these organs; lowly multicellular animals, such as the calcareous sponges, will require a relatively small number of determinants, but in the higher multicellular organisms, as, for instance, in most Arthropods, the number must be very high, reaching many thousands if not hundreds of thousands, for in them almost everything in the body is specialized, and must have varied through independent variation from the germ. Thus in many Crustaceans the smelling-hairs occur singly on special joints of the antennæ, and the number of joints furnished with a smelling-hair is different in different species; the size, too, of the smelling-hairs themselves varies greatly, being, for instance, much smaller in our common Asellus than in the blind form from the depths of our lakes, in which the absence of sight is compensated for by an increased acuteness of the sense of smell. Thus the smelling-hairs may vary transmissibly in themselves, while any joint of the antennæ may also produce one independently through variation. In this case accordingly we must assume that there are special determinants for the smelling-hairs, and for the joints of the antennæ. But we cannot always and everywhere refer identical or approximately similar organs, when there are many of them, to a corresponding number of determinants. Certainly the hairs of mammals or the scales of butterflies' wings do not all vary individually and independently, but those of a certain region vary together, and are therefore probably represented in the germ-plasm by a single determinant. These regions often appear to be very small, as is best seen by the fine lines, spots, and bands which compose the marking of a butterfly's wing, and still more in the odoriferous scales occurring in some butterflies, as, for instance, in the blue butterflies (Lycæna). These little lute-shaped scales do not occur in all species, and they occur in very unequal numbers even in those which possess them; there are certain species which exhibit only about a dozen, and these are all on one little spot of the wing. Since these odoriferous scales must have arisen as modifications of the ordinary hair-like scales, as one of my pupils, Dr. Köhler, has demonstrated by comparative studies, these ordinary hair-like scales must have varied transmissibly at certain spots, that is, their determinants have varied while those of the surrounding scales have not.
The case is the same in respect to the sound-producing apparatus of many insects. Many grasshoppers produce sounds by fiddling with the thigh of the hind leg on the wing, others by rubbing one anterior wing upon the other, and, indeed, always with one particular vein in one upon a particular vein in the other. One of these serves as the bow, the other as the string, of the violin, and the bow is furnished with teeth ([Fig. 91]), ranged beside each other in a long row, which have the same function as the colophonium of the violin, that is, to grasp and release the strings alternately, and thus to produce resounding vibrations. My pupils, Dr. Petrunkewitsch and Dr. Georg von Guaita, have recently proved that these teeth have arisen as modifications of the hairs which are scattered everywhere over the wing and leg. But only in this one place, on the so-called 'stridulating-vein,' have they been modified to form stridulating teeth (schr). Thus this vein must be capable of transmissible variation by itself alone, that is, there must be parts contained in the germ-plasm, the variation of which causes a variation solely of this individual vein and its hairs, possibly even a variation only on certain hairs on this vein.
Fig. 91. Hind leg of a Locustid
(Stenobothrus protorma), after Graber. fe,
femur. ti, tibia. ta, tarsal joints. schr,
the stridulating ridge.
On the other hand, there are also large regions, whole cell-masses of the body, which in all probability vary only en bloc, as, for instance, the milliards of blood-cells in Man, the hundreds of thousands or millions of cells in the liver and other glandular organs, the thousands of fibres of a muscle, or of the sinews or fascia, the cells of a cartilage or a bone, and so on. In all these cases a single determinant, or at least a few in the germ-plasm, may be enough. But in numerous cases it is impossible to say how large the region is which is controlled by a single determinant, and it is, of course, of no importance to the theory. In unicellular organisms the determinants will control parts of cells, in multicellular organisms often whole cells and groups of cells.
Perhaps an inference as to the nature of the determinants may be drawn from this with some probability, in as far as mere parts of cells may be supposed to have simpler determinants than whole cells and groups of cells. The determinants in the chromosomes of unicellular organisms may therefore often consist of single biophors, so that in this case the conception of biophors would coincide with that of determinants. In multicellular organisms, on the other hand, I should be inclined on the whole to picture the determinant as a group of biophors, which are bound together by internal forces to form a higher vital unity. This determinant must live as a whole, that is, assimilate, grow, and multiply by division, like every vital unit, and its biophors must be individually variable, so that the separate parts of a cell controlled by them may also be capable of transmissible variation. That they are so, every highly differentiated cell of a higher animal teaches us; even the smelling-hairs of a crab exhibit a stalk, a terminal knob, and an internal filament, and many muscle-, nerve-, and gland-cells are much more complex in structure.