THE DOGMA OF EVOLUTION 



tion the character in question has been lying latent, 

 and at last, under unknown favourable conditions, is 

 developed. "^^ This latent tendency to reversion must 

 be very strong. After only one hundred generations, 

 the proportion of blood from one ancestor is only 

 one in 2 with thirty zeroes after it. It is no wonder 

 that the breeder must preserve the purity of a strain 

 with scrupulous care, if so inconceivably small a pro- 

 portion of foreign blood can still produce an effect. 

 If this latent tendency to revert is as strong in ani- 

 mals and plants in a wild state as it is with them 

 when domesticated, and it would be difficult to deny 

 it, what chance would a variation have to be pre- 

 served, when we consider that cross-breeding with 

 others of the species which did not possess the same 

 variation is absolutely certain to occur at all times, 

 unless the variation was so advantageous, and the 

 struggle for existence was so intense, that all the in- 

 dividuals which did not have the variation were 

 killed, and all those which afterwards reverted also 

 died without progeny^ In other words, the tendency 

 to revert must be considered as universal a law of na- 

 ture as the tendency to vary. For example, even in 

 men whose choice in mating has progressed further 

 than in any other species, this reversion to a median 

 line must be very persistent for if it were not, then 

 the race would be, by this time, divided into sharply 

 distinguished characteristics, those growing taller 



21 Origin of Species, vcl. I, p. 196. 



C 220 ] 



