LECTURE XXII
SHARE OF THE PARENTS IN THE BUILDING UP
OF THE OFFSPRING
The ids are 'ancestral plasms'—The reducing division brings about a diversity of germ-plasm in the germ-cells—Bolles Lee's 'Neotaxis' even in the primordial germ-cells—Häcker's observations on the persistent distinctness of the maternal and paternal chromosomes—Identical twins—The individuality is determined at fertilization—Unequal share of the ids in the determination of the offspring—Preponderance of one parent in the composition of the offspring—Certain ids of the ancestors remain unchanged in the germ-plasm of the descendants—Struggle of the Biophors—Alternation of the hereditary sequences in the parts of the child—Reversion—Datura-hybrids—Zebra-striping in the horse—Three-toed horses—New experiments in hybridization among plants by Correns and De Vries—Xenia.
As far as the phenomena of regeneration and budding are concerned, we have not been able to do much more than bring them under a formula, which harmonizes with the germ-plasm theory. But the case is different with the actual phenomena of inheritance in the restricted sense, for instance, with regard to the transmission of individual peculiarities from parent to child. Here the theory really increases our insight and lets us penetrate deeper into the causes of the phenomena; it is here no longer a mere 'portmanteau-theory.'
We are well aware, especially from observation on ourselves, that is, on Man, that the children of a pair often resemble one another but are never alike, and that one child frequently resembles one parent, another the other, while a third may exhibit a mingling of both parents. How does this come about? Since the germinal substance of both parents is derived from that of the ovum, from which they themselves have arisen—and must therefore be the same in all the germ-cells to which they give rise—new determinants cannot be added, and old ones cannot be dropped out, and variation of the determinants, the possibility of which is granted, would still not directly bring about the familiar mingling of resemblances to the two parents, but would at most give rise to something new and strange.
Here the theory helps to elucidate matters. We found ourselves obliged to assume that the germ-plasm is composed of ids, that is, of equivalent portions of germ-plasm, each of which contains all the kinds of determinants appertaining to the building up of an individual, but each of these kinds in a particular individual form. I have already called these ids 'ancestral plasms,' and the term is appropriate, in so far that in every fertilization an equal number of ids from the father and from the mother are united in the ovum, so that the child is built up of the ids of his two nearest 'ancestors.' But as the ids of the parents are derived from those of the grandparents, and these again from those of the great-grandparents, the ids are in truth the idioplasm of the ancestors.
The expression, however, has been very frequently misunderstood, as if it were intended to mean that the ids retained unchanged for all time the character of their respective ancestors, and I have even been credited with supposing that our own ids still consist of the determinant-complexes of our fish-like or even Amœba-like ancestors. But in reality no id exactly or completely corresponds to the type, that is, to the whole being of any one of the ancestors in whose germ-plasm it was formerly contained, for each of the ancestors had many ids in his germ-plasm, and his entire constitution was not determined by any one of these alone, but by the co-operation of them all. The individual arising from a germ-cell must necessarily be the result of all the ids which make up his germ-plasm, but undoubtedly the share taken by some of them may be much stronger than that taken by others. It is also clear that, if we leave out of account any possible variation on the part of the ids, each of them belongs, not to one ancestor only, but to a whole series of ancestors, and must have taken part in their development, so that it is not the idioplasm of any particular ancestor, but only ancestral plasm in the general sense. In this sense we may quite well retain the designation, 'ancestral plasm,' for the id.
Thus, according to our view, the germ-plasm consists of ids, each of which contains all the determinants of the whole ontogeny, but usually in individually different quality.
Returning for a moment to the processes by which the reduction of the chromosomes, that is, of the nuclear rods of germ-plasm in the ovum and sperm-cell is brought about, we recall the fact that this happens at the last two divisions of the germ-cell, the so-called 'maturing divisions.' In these the nuclear substance, as we have seen, is divided between the two daughter-nuclei in a manner quite different from the usual one, for a longitudinal splitting of the rods, bands, or spheres in the equatorial plane of the nucleus does not take place, but half the number of rods move into the right and half into the left daughter-nucleus without previous division, so that in each daughter-nucleus the number of rods is reduced to half (Fig. 76).
Fig. 76. Diagram of the maturation divisions of the ovum. A, primitive germ-cell. B, mother-egg-cell, which has grown and has doubled the number of its chromosomes. C, first maturation division. D, immediately thereafter; Rk 1, the first directive cell or polar body. E, the second maturation spindle has been formed; the first polar body has divided into two (2 and 3); the four chromosomes remaining in the ovum lie in the second directive spindle. F, immediately after the second maturation division; 1, the mature ovum; 2, 3, and 4, the three polar cells, each of these four cells containing two chromosomes.
Although the distribution of the rods in this manner takes place twice in succession, the normal number is not, as we have already seen, reduced to a quarter, because, long before the occurrence of the first maturing division, a duplication of the rods by means of longitudinal division had taken place, and thus the first division differs from an ordinary division in that the splitting of the rods does not take place during the process of dividing but long beforehand. Only the second maturing division differs from all other nuclear divisions known to us, since it is not associated with any splitting of the rods at all, but conveys half of the existing rods into each daughter-nucleus. It is the time reducing division, through which the number of the rods is reduced to one half[4].
[4] Recent investigations have shown that the reduction of the chromosomes does not always take place exactly in accordance with the scheme here indicated, but that it differs from it in many cases. But as investigations on this point are as yet by no means complete, I need not go into the question further; the ultimate result is the same in any case.
This numerical reduction must, however, have other consequences; it must make the germ-cells of the same individual qualitatively unlike, that is, in relation to their value in inheritance. Let us assume only four chromosomes of the rod-form ('idants') as the nuclear elements of a species, two of which, A and B, come from the mother, and other two, C and D, from the father, the last maturing division may, as far as we can see, result either in removing the combination A and B from C and D, or A and C from B and D, or A and D from B and C; there is thus a possibility of one of six different combinations of rods in any one germ-cell. What is the same thing, six different kinds of germ-cells differing in their hereditary primary constituents may be developed in the same individual. As this new combination, or, as we may call it, neotaxis of the germ-plasm elements, takes place in female as well as in male individuals, there is a possibility that, in fertilization, 6 × 6 = 36 individuals with different primary constituents may arise from the germ-cells of the same two parents. Of course the number of possible combinations increases very considerably in proportion to the normal number of rods, for with eight of these it comes up to 70, and with sixteen to 12,870; the number of individuals differing in their inherited primary constituents would thus be enormous, for each of the 70 or of the 12,870 different hereditary minglings of the ovum could combine in amphimixis with 70 or 12,870 different sperm-cells, so that 70 × 70 and 12,870 × 12,870 offspring individually different in their primary constituents might arise from the same two parents. In Man there are said to be sixteen nuclear rods; so that in his case the last-mentioned number of parental hereditary minglings might occur. This may seem a disproportionately high number as compared with the small number of children of a human pair, but we must not judge from the case of Man alone, and in plants and animals, which we have already discussed, the number of descendants is very much larger, and is often enormous. We saw what significance this apparent extravagance on the part of nature has, for without it adaptation to changed conditions of life would not be possible, since, if only so many were born as could attain to reproduction, no selection of the fittest could take place. The same would be the case if all the young of a species were alike, and even if all the descendants of a single pair were alike, effective selection would be excluded, since only as many individualities could be selected as there were pairs of parents. It is easy to understand that selection works more effectively the larger the number of descendants of a species and the more they differ from each other. The chance that the best possible combination of characters will occur is thereby increased.
Although we cannot calculate how many individuals of different combinations of characters natural selection requires to work upon in order to direct the evolution of the species[5], we can understand that only as large a choice as possible can secure that the best possible adaptations of all parts and organs are brought about and maintained. Precisely in the fact that in every generation such an enormous superfluity of individuals is produced lies the possibility of such intensive processes of selection as must continually take place, if the adaptation of all parts is to be explained. For if among the thousands of descendants of a fertile species each hundred were alike among themselves, these hundreds would have, as far as natural selection was concerned, only the value of a single variant. But such an all-round adaptation as actually exists in the structure of species requires as many variants as possible; it requires that each individual should be a peculiar complex of hereditary characters; that is, that all the fertilized germ-cells of a pair should possess an individually well-marked character.
[5] For this reason I have left the number of id-combinations given above unaltered, though, according to the most recent researches into the processes of maturation, they are probably too high, since every conceivable combination does not actually occur. We are here concerned less with the exact number than with the principle.
The justification of this postulate becomes all the clearer if we take into consideration the male germ-cells as well as the female. Let us think of the enormous number of sperm-cells which are produced by many animals, and indeed by the highest of them—an almost incalculably large number which certainly goes far beyond millions. Let us assume that in Man there may be 12,870 million spermatozoa, then, with sixteen ids, and with an equally frequent occurrence of all possible combinations of germ-plasm—there would be 12,870—there would be a million of each type containing identical germ-plasm. The danger that several ova would be fertilized by identical sperm-cells would be by no means small.
It cannot, therefore, surprise us that other means have been employed by Nature to secure re-groupings of the ids. The simplest means would be, if before each division of the primitive germ-cells the nuclear rods were to divide, and if the split halves were irregularly intermingled, then at the formation of the next nuclear spindle an entirely new arrangement of the halves would result. But in animals, at least, this is certainly not the case; the processes of reduction are restricted to the maturing divisions.
Years ago Ischikawa observed that, in the conjugation of Noctiluca, the nuclei of the two animals become closely apposed, but that they do not fuse, although they behave like a single nucleus in the division which follows. In this case paternal and maternal nuclear substance remain separate ([Fig. 83, vol. i. p. 317]). The same phenomenon has since been repeatedly observed in many-celled animals, first by Häcker, then by Rückert in the Copepods, and afterwards by Conklin in the eggs of a Gastropod (Crepidula). But all these observations referred only to the earlier stages of ovum-segmentation up to twenty-nine cells, and it could not be affirmed that the distinctiveness of the paternal and maternal chromosomes lasted till farther on into the ontogeny. Professor Häcker now informs me, however, that he has been able to trace this separateness in a Copepod (Canthocamptus) not only from the beginning of segmentation on to the primitive genital cell, but also through the divisions of this up to the mother-egg-cell[6]. Thus we may now assume that the paternal and maternal hereditary bodies remain distinct, not only for a time, but throughout the whole development, a fact which confirms our assumption of the independence of the nuclear rods, notwithstanding their apparent breaking-up in the nuclear reticulum of the 'resting' nucleus. This new knowledge throws fresh light in another direction; it proves to us that the remarkable and complicated processes which go on in the nucleus during the maturing divisions have really the significance which I long ago ascribed to them[7], that of effecting the maximum diversity of intermingling of the paternal and maternal hereditary elements. For Häcker has shown that during the second maturation-division the paternal and maternal chromosomes are no longer united each in a special group, but occur scattered about in the nucleus, and subsequently come together again to form two differently combined groups.
[6] Since this was written Häcker has published his results. See Anatom. Anzeiger xv (1902), p. 440.
[7] See my essay, Amphimixis, Jena, 1891.
If this were not so, if the maternal and paternal chromosomes remained separate, then the reducing division would cause only one of these groups to reach each of the germ-cells, and thus each mature ovum or sperm-cell would contain either only paternal or only maternal hereditary bodies. But this would make a reversion to more than three generations back impossible, and as such reversions undoubtedly occur, we must conclude that manifold new combinations of the paternal and maternal chromosomes take place. This obviously happens during the maturation-divisions, at least in the Metazoa.
The more numerous the rods or the free individual ids in a species are, the more numerous are the possible combinations. Whether all the mathematically possible combinations actually occur is a different question, which I should not like to answer in the affirmative just yet; but in any case the actual number of combinations in a species with many nuclear elements will be greater than in one with few, and in this respect those species in which the ids occur as independent granules will have an advantage over those in which they are combined into rods or bands (idants). These latter, however, afford us a better possibility of deducing the new combinations of the ids, although the idants themselves are not outwardly distinguishable from each other.
I must refrain from going into these highly interesting processes in more detail just now. So much is certain, that Nature makes use of various means to bring about the re-combination, and at the same time the reduction of the ids during the two 'reducing divisions.' This is proved by the fact recently established by Montgomery, that in many animal groups reduction results from the first maturing division. Whether it operates at this stage with rings, bands, double rods, X-shaped structures, groups of four (tetrads), and so on, all this serves the same end, the more or less thoroughgoing re-arrangement of the hereditary vital units. I am convinced that new investigations into these processes, if they were undertaken from this point of view, would lead to very important results[8]. It would be important to find out how great the variations are which thus arise, for it is very probable that they differ in degree in the different animal-groups. Even the combination of the ids into rods (idants) indicates that some species may be more conservative than others in maintaining their id-combinations, and that there will be among them a greater tenacity in the hereditary combinations of characters (i.e. of the 'type' of the parents). If we should succeed in penetrating more deeply into these processes we should probably also understand why in certain human families the hereditary characters are transmitted more purely and more tenaciously than those of other families with which they have mingled, and so on. It may well be that the persistence of character is due to the fact that ids which have once combined into rods hold firmly together, for it seems to me in no way impossible that individual differences should occur even in these most delicate processes.
[8] Since this was written for the first edition observations of this process have been considerably increased, and discussions as to the exact interpretation of these are in full tide; we are surrounded by a wealth of new observations, facts, and explanations, without having attained to a consistent and unified theory. Several naturalists, such as Boveri, Häcker, Wilson, and others, have attempted interpretations, but these are in many points contradictory to one another. It is therefore impossible to enter into the question in detail here; further light from new observations must be awaited. So much we may say, however, that it is not chance alone which presides over the re-arrangement of the chromosomes during the reducing divisions; affinities play a part also; there are stronger or weaker attractions between the chromosomes, which aid in determining their relative position to one another.
But let us leave these more intimate questions out of account altogether, and turn our attention to the more obvious and less delicate phenomena, and we find that the re-arrangement of ids (Neotaxis) which we have just discussed affords a simple explanation of the generally observed phenomenon of the differences between individuals! Each individual is different from every other, not in the case of Man alone, but in all species in which we can judge of differences, and this is true not only of descendants of different parents, but even of those of the same parents.
Of course the differences between two brothers or two sisters do not depend entirely on the hereditary basis, but in part also on external conditions which have affected them from embryonic development onwards. Let us suppose that of two brothers who have sprung from identical germ-cells one becomes a sailor, the other a tailor; it would not surprise us to find them very different in their fiftieth year, one weather-beaten and tanned, the other pale; one muscular, straight, and vigorous, the other weakly and of bent carriage. The same primary constituents develop differently according to the conditions to which they are exposed. But the two brothers will still resemble each other in the features of the face, colour of hair, form of eyes, stature and proportion of limbs, perhaps even in a birthmark, more than any other human beings of their own or any other family, and this resemblance will depend upon the identity of the hereditary primary constituents, on the similar id-combination of the germ-plasm.
Man himself affords a particularly good example in favour of this interpretation in the case of so-called 'identical twins.' It is well known that there are two kinds of twins, those that are not strikingly alike, and often very different, and those that are alike to the extent of being mistakable for one another. Among the latter the resemblance may go so far that the parents find it necessary to mark the children by some outward sign, so that they may not be continually confused. We have now every reason to believe that twins of the former kind are derived from two different ova, and that those of the latter kind arise from a single ovum, which, after fertilization, has divided into two ova. This not infrequently occurs in fishes and other animals, and we can bring it about artificially in a number of species by experimentally separating the two first blastomeres.
We have here, then, a case of absolute identity of the germ-plasm in two individuals, for the id-combination of the two ova derived from the same process of fertilization must be exactly the same. That in such a case, notwithstanding the inevitable differences of external influences to which the twins are exposed from intra-uterine life onwards, such a high degree of resemblance should arise is a fact of great theoretical importance. From the basis of the germ-plasm theory we can very well understand it, for, according to the theory, only precisely similar combinations of ids can give rise to identical individuals.
But we learn more than this from the occurrence of identical twins. They prove above all that the whole future individual is determined at fertilization, or, to express it theoretically, that the id-composition of the germ-plasm is decisive for the whole ontogeny. It might have been supposed that the combination of ids could change again during development, and that a greater multiplication of some than of others might take place at certain stages of development, or through certain chance external influences. It might have been thought that there was a struggle among the ids in the sense that some of them were suppressed and set aside. All such suppositions break down in face of the fact of identical twins, which teaches us that identical germ-plasm evokes an ontogeny which runs its course as regularly as two chronometers, which are constructed and regulated alike.
But when I say that a struggle of the ids, in the sense of a material setting aside of some of them, cannot take place, I by no means intend to maintain that the influence which each individual id exerts on the course of development may not be disproportionate to that exerted by others, and, under some circumstances, very disproportionate indeed. I must refrain from entering into this subject in detail now, but I should like to give at least an indication of what I mean.
If the germ-plasm consists of ids, these ids collectively must determine the structure, the whole individuality—let us say, briefly, the 'type' of the offspring; it is the resultant of all the different impelling forces which are contained in the different ids. If these were all equally strong, and all operating in the same direction, they would necessarily all have the same share in the resultant of development, the 'type' of the child. But this is not the case.
Numerous experiments on the hybridization of two species of plant have taught us that the descendants of such hybridization usually maintain a medium between the ancestral species; but it is not always the case, for in many hybrids the character of one species, whether paternal or maternal, preponderates in the young plant.
We recognize the same thing still more clearly in Man, whose children by no means always maintain a medium between the characters of the two parents, but frequently resemble one—the father or the mother—much more strongly than the other.
How can this fact be theoretically explained? Must we ascribe to the ids of the father or of the mother a greater determining power? Without excluding such an assumption as on a priori grounds inadmissible, I am inclined to believe that we do not require it to explain this phenomenon. For, if we take our stand simply on the fact of the preponderance of one parent, it follows directly from this that not all the ids control the type of the child, let the cause of the non-co-operation of some of them be what it may. But if in this case only a portion of the ids contained in the germ-plasm controls the type, this combination of ids suffices to make the child resemble one parent, the father, for instance, and consequently half the number of ids is sufficient in some circumstances to determine the child—taking for granted that the one-sidedness of the inheritance is complete, which never actually happens. But the half number of ids can only suffice if it includes the same combinations of ids which have determined the type in the case of the father; as soon as one or more ids of this particular combination are replaced by others the paternal germ-plasm alone is not enough to call forth complete resemblance in the child.
But, at the reduction, a change of arrangement of ids takes place, and a new combination arises, and thus each germ-cell receives its particular group of ids. It may thus happen that, in one particular sperm-cell, exactly the same group of ids is contained as that which determined the type of the father, and that the same is true of a particular egg-cell in regard to the type of the mother. Let us now assume that a sperm-cell and an egg-cell meet, which contain both those groups of ids which had determined the type of the father and of the mother; if the determining power of the maternal and paternal ids were equal a child would result which would maintain the medium between father and mother.
As is well known this does happen not infrequently, although it is difficult or impossible to demonstrate it precisely. In plant-hybrids proof is easier, and it has been established that by far the greater number of hybrids maintain a medium between the characters of the two ancestral species. This proves that our assumption of equal strength of the ids of both species must be correct in general, for we know definitely in this case, as I shall show later, that the paternal and the maternal ids are equivalent as regards the characters of the species. This is the case, for instance, with the hybrids between the two species of tobacco-plant, Nicotiana rustica and N. paniculata, which were reared by Kölreuter as far back as the eighteenth century, and which then, as now, maintained a fairly exact medium between the two ancestral species, and did so in all the individuals. Both species thus strive to stamp their own character on the young plant, and in both the hereditary power is equally great; in both it is contained in the same number of ids, that is, in the half, for both kinds of sex-cell have undergone reducing division. We have here, then, strict proof that the half number of ids suffices to reproduce in the offspring the type of the species, or, more generally, of the parents.
If we apply these results to the inheritance of individual differences in Man, we may say, that those germ-cells, to which at the reducing division the same combination of ids has been handed on as that which already determined the type of the parent, will endeavour to impress this image again on the child. If a female cell of this kind combine with a male which likewise contains the facies-combination of the parent, in this case the father, the same thing will happen which we described in the case of the plant-hybrids, that is, a medium form between the type of the two parents will arise.
Not infrequently, however, there is a marked preponderance of the one parent in the type of the child, and we have to inquire whether the theory gives us any help with regard to such a case.
One might be inclined to assume a difference in the determining power of the paternal and maternal ids, but if we cannot show to what extent and for what reason this power may be different such an assumption remains rather an evasion than an explanation. Moreover, it would not always apply to the conditions in Man, for if, for instance, the ids of a particular mother were in general stronger than those of the father, all the children of the pair in question would necessarily take after the mother; but it happens not infrequently that one child resembles the father preponderantly, and another the mother. Moreover, the ids pass continually from the male to the female individual, and conversely, by virtue of the continuity of the germ-plasm, so that the idea that sex can have anything to do with the relative strength of the ids is altogether erroneous.
But, as I have already said, unilateral inheritance occurs even in the mingling of species-characters, and most clearly in the case of plant-hybrids. Thus, for instance, hybrids between the two species of pink, Dianthus barbatus and Dianthus deltoides, resemble the latter species much more closely than the former, and the hybrid between the two species of foxglove which are wild in Germany, Digitalis purpurea and Digitalis lutea, is much more like the latter than like the former.
It might be reasonably asked whether, in these crossings, the normal number of ids in one species is not greater than in the other. We know that, among animals at least, differences in the normal number of chromosomes occur even in very nearly related species. It is not impossible that this, in many cases, is really the cause of the diversity of transmitting power in different species. Nevertheless, we cannot rest satisfied with this, for, in the first place, this cause could not apply to the apparent unilateral inheritance from one parent in Man, since the normal number of ids, as far as we know, is strictly maintained in the same species, and second, this would not explain certain phenomena of inheritance in plant-hybrids.
It happens not only frequently, but usually, that the different parts of the hybrid take after one or other parent in different degree, and this is the case also with children. In the hybrid between the two species of tobacco-plant, Nicotiana rustica and N. paniculata, which I have already given as an example of a medium form between the two parents, such diversities occur regularly in all the hybrid individuals. Thus the corolla-tube of the hybrid is nearer N. paniculata in regard to its length, but nearer N. rustica in regard to its breadth. Many hybrids suggest one parent-form in the leaves, the other in the blossoms. In the same way in the child the form of eye may be that of the father, the colour of the iris that of the mother, the nose maternal, the mouth paternal—in short, the preponderance in heredity swings hither and thither from part to part, from organ to organ, from character to character, and this is even the rule though the oscillations may not always be apparent and are often invisible.
If we think of the proposition we arrived at earlier, and which was proved chiefly by the case of identical twins, that the facies or 'type' of the descendant is determined at fertilization, we may be inclined to regard such an oscillation of the hereditary tendencies as almost impossible, for it means that, with the given mingling of parental germ-plasms, the potency of inheritance from the two parents in every part of the offspring is determined once for all in advance. But the case of identical twins corroborates these oscillations, for in them, too, the father predominates in one part, the mother in another, and it proves, at the same time, that these oscillations do not depend on any chances whatever in development, but that they are exactly predetermined in the mingling of the hereditary substances in the germ-plasm of the fertilized ovum, and are strictly adhered to throughout development.
This fact can only be explained thus: the primary constituents of the different parts and characters of the body are contained in the parental germ-plasm in varying degrees of hereditary or transmissive strength, and this can be understood very well from our point of view without putting anything new into the 'portmanteau' of our theory (Delage).
But I must digress a little in order to make this plain.
When, in speaking of plant-hybrids, I said that the collective ids of the germ-plasm of a species must be equivalent in regard to the characters of the species, I did not speak quite precisely; in the majority of ids, in many cases in an overwhelming majority, this must be the case, but not actually in all, at least not on the assumption we make that the transformation of species is accomplished under the control of natural selection.
Let us recall what we have already established in regard to the evolutionary power of natural selection, namely, that the changes which it controls can never transcend the range of their utility, and it will be clear to us that, of the many ids which make up the germ-plasm of the species, only so many will be modified as are necessary to evoke the character which has varied. Just as the protective resemblance of an insect to a leaf may be raised to a very high pitch, but can never become perfect, because an imperfect resemblance is already sufficient to deceive the persecutor, and the selective process comes to a standstill because individuals which possessed a still greater resemblance to a leaf would be no better protected from destruction than the others, so in the modification of a species the whole of the ids need not at once be modified, if a majority is sufficient to stamp the great majority of individuals with the desired variation. But it may happen that, at the reduction of ids during the development of the germ-cells, an id-combination with wholly or almost wholly unchanged ids may come together in one germ-cell, and if another sperm-cell of this kind meets with an egg-cell similarly constituted, an individual of the old species must arise. But this must—on our assumption—be at a disadvantage as compared with the transformed individuals in the struggle for existence, and will perish in it, and therefore the number of unmodified ids in the germ-plasm of the species will gradually diminish. It is obvious, however, that this will take place very slowly, as we may conclude from the phenomena of reversion, of which I shall have to speak later on.
But what is true of the ids is true also of their constituent parts, the determinants, and that—if I mistake not—is fundamental in the interpretation of the alternation of hereditary succession in the parts of the child.
According to our theory, the ids do not collectively exert a controlling influence on the cells, not even on the germ-cells, whose histological differentiation into spermatic or egg-cells can only depend on control through specific sex-cell determinants. It is the different determinants of the ids that control; transformations of the species will, it is true, depend on transformations of the ids, but this need not necessarily consist in a variation of all the determinants of the id. If, for instance, two species of butterfly, Lycæna agestis in Germany and Lycæna artaxerxes in Scotland, only differ from each other in that the black spot in the middle of the wing in L. agestis is milk-white in L. artaxerxes, no other determinants in the id of the germ-plasm can be different except those which control this particular spot. In a majority of the ids in L. artaxerxes the determinants of this spot must have been modified, let us say, to the production of 'milk-white.' This majority will increase very slowly if the white colour has no pronounced advantage for the persistence of the species, but it will increase gradually, as we have already seen, though extremely slowly, through the elimination of those individuals whose germ-plasm at the reducing division has chanced to receive a majority of ids with the old, unmodified determinants, and which have therefore reverted to the ancestral form. This will happen whenever the new character has any use, however small, in maintaining the species.
But in most modifications of species quite a number of parts and characters have undergone variation either simultaneously or in rapid succession; in many cases nearly all the details of structure, and therefore almost all the determinants of the germ-plasm, must have varied. We must not, however, assume that all the equivalent determinants, for instance, all the determinants K in all the ids, have varied[9], and above all we must not take for granted that the determinants of different characters or parts of the body, for instance, the determinants L, M, or N, must all undergo variation in an equal number of ids. It will depend on two factors whether a new character is implicit, in the form of varied determinants, in a small or in a very large majority of ids: first, on the relative age of the character, and, secondly, on its value in relation to the persistence of the species. The more important a character is for the species the more frequently is it decisive for the life or death of the individual, and the more sharply will individuals not possessing it be eliminated, and the more rapidly, therefore, will those whose germ-plasm still contains a majority of the unvaried determinants of this character tend to disappear. In this way these determinants will tend to sink down from generation to generation to an ever smaller minority in the germ-plasm of those that survive.
[9] By equivalent or homologous determinants I mean the determinants of different ids which determine similar parts, e.g. the scales of that wing-spot in Lycæna agestis which is alluded to above, and to which we must refer again in more detail.
Thus in the ids of any species which has been in some way transformed—and that is as much as to say, in every species—the equivalent or homologous determinants are modified in a very varied percentage. A very modern and at the same time not very important character K´ will only be contained in a small majority of ids, while in the remainder the original homologous ancestral determinant K is contained; an older but not very much more important character M´ must have its determinants in a larger majority in the ids, while a character V´ of decisive importance for the preservation of the species, if it has been in existence long enough, must be represented in almost all the ids, so that the homologous unvaried determinants of the ancestral species V can only have persisted in an id here and there.
If this argument be correct, many phenomena of inheritance become intelligible, especially the variability in the expression of the inheritance in the parts of the offspring, which is more or less rigidly predetermined at fertilization. For the germ-plasm thus contains in advance every kind of determinant in diverse nuances, and in definite numerical proportions. In a plant N´, for instance, Ba´ may be the determinant of the modern leaf-form, and may occur in twenty-two out of twenty-four ids of the germ-plasm, while the two remaining ids still contain unmodified the old leaf-form determinants Ba, which the ancestral form N possessed. But the flower of N´ may be of still more recent origin, and contain the modern flower determinants Bl´ only in sixteen out of twenty-four ids, while in the other eight the old flower-determinant Bl of the ancestral form has persisted. Let us now suppose that another nearly related species P´ has, conversely, a recently changed leaf-form but a very ancient form of flower, so that the former is represented only in sixteen ids by the determinants of the leaf ba´ and the latter in twenty-two ids by the flower-determinants bl´: it is obvious that when the two species are crossed, notwithstanding the equal number of ids in the germ-plasm, the leaves of the hybrid will resemble more closely those of the ancestral form N, and the flowers those of the form P; it is even conceivable that in such a case the numerically preponderating leaf-determinants N, and the equally preponderating flower-determinants of P may form a close phalanx, so to speak, against the much less numerous homologous determinants of the other species, and that against this power working in a definite direction the others can make no headway and are simply condemned to inaction.
How we may or can picture this as occurring is a question which of course admits only of being answered very hypothetically, and it leads us, moreover, into the region of the fundamental phenomena of life, with the interpretation of which we are not here concerned. For the present we have assumed that life is a chemico-physical phenomenon, and we have postponed the deeper explanation of it to the remote future, that we may confine ourselves in the meantime to the solution of the problem of inheritance on the basis of the forces resident in the vital elements. But we may, nevertheless, make the supposition that a kind of struggle between the different kinds of biophors may take place within the cell, if the homologous determinants of all the ids for the control of the cell have entered into it.
In many cases this struggle will be decided by the numerical preponderance of one kind of determinant over the other, but it is certainly conceivable that dynamic differences may also have something to do with it.
Let us, however, abstain from trying to penetrate further into the obscurity of these processes, and let us content ourselves with establishing that the preponderance of one parent in some or many parts of the child may be almost if not quite complete, and that this compels us to assume that the hereditary substance of the other parent is in such cases rendered inoperative—for we know it is present—since the ids of both parents all go through the whole ontogeny, and are contained in every somatic cell.
Upon this struggle between homologous determinants depends the possibility of the entire suppression or inhibition of the influence of one parent, the whole diversity in the mingling of paternal and maternal character in the body of the offspring. It is in this that we must seek the explanation of the fact that not only whole bodily parts of the child, such as arms, legs, the nature of the skin, the form of the skull, may take after sometimes the father, sometimes the mother wholly or predominantly, but that the small separate subdivisions of a complex organ may sometimes turn out more maternal, sometimes more paternal. Thus intelligence from the mother and will from the father, musical talent from the father and a talent for drawing from the mother, may be inherited by the same child. I do not doubt that genius depends in great part on a happy combination of such mental endowments of the ancestors in one child. Of course something more is necessary, namely, the strengthening of certain of these hereditary endowments, but of this we shall speak later.
It is, however, not only the immediate ancestors, that is to say, the parents, that have to be taken account of in this mingling of hereditary contributions, but also those more remote. Not a few characters in the child do not occur in either parent, but were present in the grandparents, and their reappearance is called 'atavism' or 'reversion.' Let us consider this phenomenon in more detail, and try to find out whether and how far it can be interpreted by means of our theory.
The simplest and clearest cases are again found among plant-hybrids. It may happen, for instance, that a hybrid between two species, when dusted with its own pollen, gives rise to descendants, some of which resemble only one of the ancestral forms: thus we have reversion to one of the grandparents. The explanation of this lies in the different modes in which the reducing divisions are effected; if they take place in such a manner that all the paternal ids of the hybrid are separated from the maternal ids, then the result is germ-cells which are like those of the grandparents, that is, those of the parent species, and these, if they happen to combine in amphimixis, must give rise to a pure seedling of one or other of the two ancestral species. This case occurs less rarely than was formerly supposed, and than it could do if absolutely free combination of the idants took place at reduction. If combination were quite unrestricted, all other possible combinations would be likely to occur as frequently as these. But recent experiments have shown that, in many plant-hybrids, the germ-cells of the hybrids which are fertilized by their own pollen are either purely paternal or purely maternal. There cannot, therefore, be free combination of the idants at the reducing divisions; the idants of the two parent-forms separate from one another and do not combine. It is doubtful, however, whether the same thing occurs within a race, for instance, in the case of reproduction within a human race.
In Man reversion to a grandparent occurs not infrequently, and we may explain it thus: the id-group which controlled the type of the grandfather was also contained in the germ-cell which gave rise to the existence of the father, but it did not dominate the type in that case because a more powerful id-group was opposed to it in the germ-cell of the grandmother. When, later on, at the reducing divisions of the germ-cells of the father, this id-group again arrived in the sperm-cells of the father, it would predominately control the type of the child, that is, of the third generation, provided that the egg-cell with which it combined contained a weaker id-group.
In the case of ordinary plant-hybrids what are designated reversions can only be called so in a wide sense, for the ancestral characters are contained visibly in the parent, although mingled with those of the other parent. In human families, however, there are undoubted cases in which one or more characters of the grandparent reappear in the child which were not in any visible way expressed in the parent, and must therefore have been contained in the parent's germ-plasm in a latent state. And there are both in animals and plants reversions to ancestors lying much further back, to characters and groups of characters which have not been visible for many generations, and the occurrence of these can only be explained on the assumption that certain groups of ancestral determinants have been carried on in the germ-plasm in too small a number to be able to give rise ordinarily to the relevant character. Such isolated determinants may, however, in certain circumstances be strengthened by the amphimixis of two germ-cells both containing small groups of them, and thus augmented they may gain a controlling influence. In this case the chances of the reducing divisions have a part to play, since they bring together the old unvaried ancestral determinants which, as we have seen, may persist in the germ-plasm of any species through a long series of generations. This would, of course, only suffice to bring about a reversion if the determinants of the ancestral species were still contained in the germ-plasm in comparative abundance. If this is no longer the case, something more is necessary, and that is the relative weakness of the more modern determinants.
If two white-flowered species of thorn-apple, Datura ferox and Datura lævis, be crossed, there arises a hybrid with bluish violet flowers and brown stalks instead of green. This was interpreted by Darwin as a reversion to violet-flowering ancestral species on both sides, for there are even now a great number of species of Datura with violet flowers and brown stalks. When the two white forms are crossed the reversion takes place every time, not merely in some cases, and we may conclude from this that in both these species there is still such a strong admixture of the same unvaried ancestral ids that they always excel the ids of the two modern species crossed, in strength though certainly not in number. And this superiority must again depend on the fact that similar determinants of the same part are cumulative in their effect, while dissimilars are not.
For this reason reversions to remote ancestors occur readily when species and breeds are crossed, while they are rare in the normal inbreeding of a species. The reversions of the breeds of pigeon to their wild ancestral form, the slaty-blue rock-pigeon, never result, as Darwin showed, and as we have already noticed, from pure breeding of one race, but only when two or more breeds are repeatedly crossed with one another. Even then it occurs by no means always, but only now and again. The germ-plasm of the breeds must therefore still contain ids of the rock-pigeon, but in a small number, varying from individual to individual. If by fortunate reducing divisions and the meeting together of a sperm-cell rich in ancestral ids with a similarly endowed egg-cell the number of ancestral ids be raised to such a point that it exceeds the number of modern-breed ids contained in each one of the conjugating germ-cells, the ancestral ids control the development and reversion occurs, for the ancestral ids together have a cumulative effect while the ids of the two parent breeds are different and therefore, as far as they are so, cannot be co-operative in their influence. But it must be understood that they need not be different as far as all their determinants are concerned, but usually only as regards some groups, and thus it happens that reversion does not occur in regard to all, but only in regard to particular characters—thus in the Datura hybrids, chiefly in regard to the colour of the flowers and the stem, and in the hybrids between different breeds of pigeon mainly in regard to the colour and marking of the plumage.
The reversions of the horse and ass to striped ancestors, which Darwin has made famous, go much further back into the ancestral history of the species, for while we know the ancestral form of the domestic pigeon in the still living rock-dove (Columba livia), the ancestral form common to the horse and the ass is extinct, and we can only suppose that it was striped like a zebra, because such striping occasionally occurs in pure horses and pure asses, at least in their youth, although now only on the legs, and because this striping is often more marked in the hybrid between the horse and the ass, the mule. In Italy, where one sees hundreds of mules, the striping is not exactly frequent, but it may occur in about two per cent., while in America it is said to be much more frequent. The germ-plasm of the horse and the ass must therefore contain, in varying numbers, ids whose skin-colour determinants represent in part still unmodified ancestral characters. When two germ-cells chance to meet in fertilization, both of which have received, through a favourable reducing division, a relatively large number of such ids, a relative majority of these in the fertilized ovum is opposed to the dissimilar and therefore mutually neutralizing homologous determinants of horse and ass, and reversion to the ancestral form occurs.
These cases of reversion are enough to show us that the old unmodified ancestral determinants may persist in the germ-plasm through long series of generations. But an even deeper glimpse into the dim ancient history of our modern species of horses is afforded by the occurrence of three-toed horses, references to a small number of which the palæontologist Marsh was able to discover in literature, and one of which he was able to observe in life. Julius Caesar possessed a horse whose three-toed feet represented a reversion to the horses of Tertiary times, Mesohippus, Miohippus, and Protohippus or Hipparion; for all these genera possessed, in addition to the strong middle toe, two weaker and shorter lateral toes.
In the germ-plasm of our modern horses there must still persist in certain ids the determinants of the ancestral foot, which, after a long succession of favourable reducing divisions combined with favourable chances of fertilization, may come to be in a majority, and may thus be able to induce a reappearance of a character which has long been hidden under the surface of the present-day type of the species.
I do not propose to enter further on the discussion of the phenomena of inheritance. A more detailed investigation of the phenomena of reversion is to be found in 'The Germ-plasm' published ten years ago; and the discussion could not be resumed here without a critical consideration of a relatively large series of newly acquired facts not always harmonizing, and, as yet, not even fully available. The year 1900 has given us the investigations of three botanists, De Vries, Correns, and Tschermak, who have sought by experiments in hybridization between different sorts of peas, beans, maize, and other plants, to throw light on the phenomena of inheritance, and thus on the actual processes which occur in the germ-plasm at the reducing division. This led to the discovery that similar experiments had been published as far back as 1866 by the Abbot of Brünn, Gregor Mendel, and that these had been formulated as a law which is now called Mendel's law. Correns showed, however, that this law, though correct in certain cases, did not by any means hold good in all, and we must thus postpone the working of this new material into our theory until a very much wider basis of facts has been supplied by the botanists. There is less to be hoped for from the zoologists in regard to this problem owing to the almost insuperable difficulties in the way of a long series of experiments in hybridization in animals. I myself have repeatedly attempted experiments in this direction, and have always had to abandon them, either because the crossing succeeded too rarely, or because the hybrids did not reproduce among themselves, or did so defectively, or because the distinguishing characters of the crossed breeds proved insufficiently tenacious or diagnostic. But it would be a fine task for zoological gardens to undertake such experiments from the point of view of the germ-plasm theory, and their success would afford material for the criticism of the theory, the more valuable because it is apparent from the experiments on plants that the processes of heredity are manifold, and are far from being uniform in different domains[10].
[10] Castle and Allen have recently published the results of experiments in crossing white mice with grey, and these confirm Mendel's Law.
I have assumed for my theory that the reducing division took place according to the laws of chance, and that thus every combination of ids occurred with equal frequency. This assumption seems to be confirmed, by the experiments of the botanists I have mentioned, only in so far that in the crossing of hybrids with one another every combination of distinctive characters occurred with equal frequency. But, on the other hand, the splitting of the germ-plasm at the reducing division seems, as I said before, in many cases to take place in such a way that the id-groups of the two parents are discretely separated from one another; this was so in the stocks, peas, beans, and other hybrids. But even if this were always the case in these, we could hardly infer that it must be the same everywhere; we should rather expect that the relationship of the two parents and their ids would bring into play the finer attractions and repulsions between the ids of the germ-plasm, and would thus determine their arrangement and grouping. Further investigations may clear up this point; in the meantime we can only say that already—even among hybrids—many deviations from Mendel's Law have been established, for instance, by Bateson and Saunders (1902).
Before I conclude this lecture I should like to refer briefly to a phenomenon which Darwin was acquainted with and sought to explain through his theory of Pangenesis, but which at a later date was regarded as not sufficiently authenticated to justify any attempt at a theoretical explanation, since it seemed to contradict all our conceptions of hereditary substance and its operations. I refer to the phenomenon to which the botanists have given the pretty name of Xenia (guest-gifts), and which consists in the fact that in crosses of two different plants the characters of the male may appear not only in the young plant but even in the seed, so that a transference of paternal characters seems to take place from the pollen-tube to the mother, to the 'tissue of the maternal ovary.' In heads of yellow-grained maize (Zea) it is said that, after dusting with pollen from a blue-seeded variety, blue seeds appear among the yellow, and similar observations on other cultivated plants have been on record for more than half a century. Thus dusting the stigma of green varieties of grape with the pollen of a dark blue kind is said frequently to give rise to dark blue fruits.
Darwin accepted these observations as correct, and endeavoured to explain them as due to a migration of his 'gemmules' from the fertilized ovum into the surrounding tissue of the mother-plant. His explanation was not correct, we can say with confidence now, but he was right so far, for the phenomena of Xenia do occur; they are not illusory as most modern botanists seem inclined to believe. I myself was at first inclined to wait for further facts in proof that the phenomena of Xenia really occurred before attempting to bring them into harmony with my theory, and this will not be found fault with when it is remembered that these cases of Xenia seem to stand in direct contradiction to the fundamental postulates of the germ-plasm theory. For this depends essentially on a definite stable structure of the germ-substance, which lies within the nucleus in the form of chromosomes, and which cannot pass from one cell to another in any other way than by cell-division and division of the nucleus; how then could it pass from the fertilized ovum to the cells of the endosperm which do not derive their origin from it at all, but from other cells of the embryo-sac? In point of fact some of my opponents have cited Xenia as an actual refutation of my theory.
Fig. 82. Fertilization in the Lily (Lilium martagon), after Guignard. A, the embryo-sac before fertilization; sy, synergidæ; eiz, ovum; op and up, upper and lower 'polar nuclei'; ap, antipodal cells. B, the upper part of the embryo-sac, into which the pollen-tube (pschl) has penetrated with the male sex-nucleus (♂k) and its centrosphere; below that is the ovum-nucleus (♀k) with its (also doubled) centrosphere (csph). C, remains of the pollen-tube (pschl); the two sex-nuclei are closely apposed. Highly magnified.
That cases of Xenia really do occur is now established by the comprehensive and at the same time exceedingly careful experiments recently made by C. Correns with Zea Mais; it is only necessary to look through the beautiful figures with which his work is adorned to be convinced that heads of maize whose blossoms have been dusted with the pollen of a different kind produce more or less numerous seeds of the paternal kind, usually mingled with those of the maternal. Thus heads of the variety Zea alba resulting from fertilization with Z. cyanea exhibited a majority of white grains, but among them a smaller number of blue; and the converse experiment, of dusting Z. cyanea with the pollen of Z. alba, yielded heads in which a minority of white grains appears among a majority of blue. But it is always only the nutritive layer surrounding the embryo—the endosperm—which exhibits the character of the paternal species, and even the capsule surrounding the seed shows nothing of it, but is purely maternal. Thus the heads of different species with pale-yellow capsule, when dusted with the pollen of Z. rubra, never have red seeds like those of Z. rubra, but always seeds with a pale-yellow skin, while, in the converse experiment, dusting of the red-skinned species Z. rubra with pollen from Z. vulgata, all the seeds are red, like those of the maternal species, and the influence of the paternal species only shows when the strong red skin has been removed, so that the intense yellow colour of the endosperm, which in the pure maternal species is white, is exposed to view. Thus the mysterious influence of the pollen never goes beyond the endosperm, and the riddle of this influence is solved in the most unexpected manner, indeed was solved even before Correns had securely established the genuine occurrence of Xenia. The explanation is due to recent disclosures in regard to the processes of fertilization in flowering plants.
It had long been known that the pollen-tube contains not merely one generative nucleus but two, which arise from one by division. But what had till recently remained unknown was that not only one of these penetrated into the embryo-sac to enter into amphimixis with the egg-cell, but that the other also makes its way in, and there fuses with the two nuclei which had long been designated the upper and lower polar-nuclei ([Fig. 82, op. cit.]). Nawaschin and Guignard demonstrated that these two nuclei fuse with the second male nucleus; thus two acts of fertilization are accomplished in the embryo-sac, and one of these gives rise to the embryo, while the second becomes nothing less than the endosperm, the nutritive layer which surrounds the embryo, whose origin from the polar nuclei had been previously recognized.
Thus the riddle of Xenia is essentially solved. We understand how paternal primary constituents may find their way into the endosperm, and indeed must do so regularly; we understand also how the paternal influence never goes beyond the endosperm. The riddle is thus not only solved, but at the same time the view which assumes a fixed germ-plasm, and believes it to lie in the nuclear substance of the germ-cells, receives further confirmation, if it should need any, for if facts which are apparently contradictory to a theory can be naturally brought into harmony with it, this affords a stronger argument for the correctness of the theory than the power of explaining the facts which were used in building it up.
There is much more to be said in regard to Xenia, and I am sure that much that is of interest will be brought to light by deeper investigation; theoretical difficulties will still have to be overcome, and I have already pointed out one of these in my 'Germ-plasm,' but I must here rest satisfied with what has been already said.
We have now passed in review and attempted to fit into the theory a sufficiently large number of the phenomena of heredity for the purpose of these lectures. Although, as is natural, much of this must remain hypothetical, we may accept the following series of propositions as well founded: there is a hereditary substance, the germ-plasm; it is contained in very minimal quantity in the germ-cells, and there in the chromosomes of the nucleus; it consists of primary constituents or determinants, which in their diverse arrangement beside or upon one another form an extremely complex structure, the id. Ids and determinants are living vital unities. Each nucleus contains several, often many, ids, and the number of ids varies with the species and is constant for each. The ids of the germ-plasm of each species have had a historical development, and are derived from the germ-plasm of the preceding lineage of species; therefore ids can never arise anew but only through multiplication of already existing ids.
And now, equipped with this knowledge, let us return to the point from which we started, and inquire whether the Lamarckian principle of evolution, the inheritance of functional modifications, must be accepted or rejected.