The Evolution Theory, Vol. 2 of 2

by August Weismann

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.