Weismannism not only retained the principle of utility and selection, but made it the only principle, rejecting entirely the action of external conditions as a cause of congenital modifications, _i.e._ of characters whose development is predetermined in the fertilised ovum. It is to Weismann that we owe precise and definite conceptions, if not of the nature of heredity, at least of the details of the process. From him we learned to think of the ova or sperms, of the reproductive cells or 'gametes' of an individual, as cells which were from an early stage of development distinguished from the cells forming the organs and tissues; to regard the organism as consisting of soma on the one hand and gametes on the other, both derived from the original zygote cell, not the gametes from the soma. Weismann saw no possibility of changes induced by any sort of stimulation in the soma affecting the gametes in such a way as to be redeveloped in the soma of the next generation. He attributed variation partly to the union of gametes containing various determinants, which he termed amphimixis: this, however, would introduce nothing new. Then he proposed his theory of germinal selection, determinants growing and multiplying in competition, some perhaps disappearing altogether, though this does not satisfactorily account for entirely new characters. With Weismann, however, every species was a different adaptation, and natural selection was the _deus ex machina_; to quote his own words, _Alles ist angepasst_. Romanes and other writers, on the other hand, had always maintained that in many cases the constant peculiarities of closely allied species had no known utility whatever, so that the problem presented by these characters was not explained by any theory of the origin of adaptations. Mendelism, since 1900, has studied experimentally the transmission of definite characters, and maintains that the characters of species are of the same nature as the characters which segregate in Mendelian experiments. Such characters are not in any way related to external conditions, and cannot, therefore, be adaptive except by accident. Professor Bateson goes so far as to admit that such large variations or mutations offer more definite material to selection than minute variations too small to make any important difference in survival, but as regards species the important factor is the occurrence of mutations which are inherited and at once form a distinct definite difference between allied species which is not due to selection and has nothing to do with adaptation.