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.