170 Experimental Zoology 



In the first place, if we are to study those characters that be- 

 have, as a rule, as units, we can find for this purpose no better 

 material than some of the domesticated races that differ from 

 each other by single unit-characters. We are deahng here with 

 the simplest cases of discontinuous variation, and the simpler the 

 problem the better the opportunity to study it. In so far as the 

 contamination due to previous hybridization is concerned there is 

 introduced, it is true, a compHcation (but one that can be dealt 

 with, in most cases), for the unit-characters, as such, are not 

 necessarily affected by the latent characters, but can be studied 

 independently of them. Hence it is not a serious difficulty to 

 find that previous contamination has occurred. To ignore this 

 point shows a misconception of the problem of the heredity 

 of discontinuous variation. 



It is important, in this connection, to bear in mind that the same 

 rule of discontinuous heredity has been found to be true for wild 

 forms also that differ from each other by a single character. 

 Furthermore, how do we know that wild species are not also 

 hybrids? If evolution has taken place by mutation, then it is 

 possible that many wild species are complex hybrids, even if all 

 of them are not. There will also be recalled in this same connec- 

 tion de Vries's conclusion that even his elementary species of 

 Oenotheraemust also have arisen by hybridization, since it is im- 

 probable that a mutating germ-cell should meet another of its 

 kind. Therefore, until it is more evident that wild species 

 are not hybrids, this side of the argument carries Httle weight. 



It is sometimes said also that wild species that have bred true 

 for hundreds (or thousands) of generations are much purer than 

 our ''pure" domesticated races that have been bred for a rela- 

 tively small number of generations. This may be true, provided 

 it can be shown that a hybrid is purer the longer it is inbred. 

 There is little direct evidence that this is the case, but the state- 

 ment is likely to pass unchallenged because it seems so plausible ! 



Finally, if evolution has taken place by single steps, our first 

 problem is to study the heredity and results of crossing of these 

 single steps. Most wild species must differ from each other by 



