[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.