ANIMAL EVOLUTION 33 



many characters it is necessary that the substances 

 contained in the nucleus should act on those contained 

 in the cytoplasm to produce the character in question 

 in the adult, and that it may come about that one or 

 other of these substances is absent and so the character 

 in question is not produced, but will be if the two sub- 

 stances are brought together in fertilization. 



But all this is as yet unproved, and it will require 

 years of research to elucidate the many problems pre- 

 sented by the constitution of the germ-cell, and the 

 manner in which organ-producing factors are combined 

 or split up in the course of reproduction. 



This much, however, is clear, that Herbert Spencer's 

 conception of the constitution of germ-cells no longer 

 holds good. 



The germ is proved to be not a simple relatively 

 homogeneous substance, which acquires increasing hetero- 

 geneity in the course of development, but an exceedingly 

 complex heterogeneous substance, containing what, for 

 want of more definite knowledge, we must call factors, 

 and these factors are derived by inheritance from germs 

 containing like factors. By intercrossing the factors 

 may be combined, separated, added to, or subtracted, 

 and thus variations of definite kinds may be produced ; 

 but the characters to be produced are not determined 

 by the interaction of the constituent parts during the 

 development of the individual, but by the bringing 

 together or separation of the factors during the processes 

 of maturation of the germ-cells and their subsequent 

 union in fertilization. 



We conclude that for every species of animals or plants 

 — and by a species we mean a number of individuals 

 capable of interbreeding freely — there is a certain stock 

 of factors which, separately or in combination, are 

 capable of giving rise to all the specific characters, and 

 also to aU the varietal characters, manifested by the 

 species. The factors necessary for the production of 



£ 



