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.