The Evolution Theory, Vol. 2 of 2

by August Weismann

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.