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.