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 dealing 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 complication (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 thesame 
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 
Oenothere must 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 little 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 
