31 8 THE GERiM- PLASM 



case of a majority of them, which is sufficient to ensure the occur- 

 rence of the modification in question in every ontogeny, — z>., 

 in every individual of the species. This is all that is required, 

 and consequently the processes of selection cannot accomplish 

 more. After every transformation of the body in the process 

 of development of the species, the germ-plasm will thus con- 

 tain some unmodified determinants in addition to those which 

 have been transformed in any part, and these will only disappear 

 very gradually in the course of the further history of the species. 



The existence of the material by means of which reversions 

 to all specific characters operate, can therefore be proved 

 on the basis of the theory of selection : every germ-pias7n inust 

 contain a larger or smaller number of old deter7ni7iants corre- 

 spondi?ig to the characters of the ancestral species . 



The solution of the second problem, as to why these ' ancestral 

 determinants ' always exert their influence at the right spot in 

 the body, naturally follows from our theory, in which the 

 mechanism of ontogeny is referred to the gradual disintegration 

 of the germ-plasm. 



The third problem then only remains : how can the ancestral 

 determinants, which are prese7it i7i a 77ii7writy, gai7i co7itrol over 

 the 77iajority of yoimger ones ? 



We have seen that reversions to the ancestral form occur in 

 the offspring of hybrid plants, — even if they are fertilised by 

 their own pollen, — when those cells to which the e7itire group 

 of idants of the ancestral species of the sa7ne parent has been 

 distributed at the reducing division happen to come together in 

 the process of fertilisation. Whenever the germ-plasm of the 

 fertilised egg-cell contains idants of the species A alone, an 

 organism of the species A can alone arise. But this occurrence 

 is out of the question in cases of reversion to the characters of 

 remote ancestors. The germ-plasm can then never consist 

 entirely of idants of the ancestral species ; and, in fact, we may 

 doubt whether entire ancestral idants exist at all in the germ- 

 plasm of any individual of a long-established species. For the 

 number of idants (nuclear rods) is not extremely large in any 

 species ; and if, as in the instance given above, we assume this 

 number to average sixteen, even the proportion of 07ie unmodi- 

 fied to fifteen modified idants would be rather large, and would 

 render an occasional reversion to the ancestral form possible 

 in the ordinary reproduction of the species. Fertilisation need 



