Raymond Pearl 
273 
which might supphxnb its parent form. But in order to secure diversitij, multi- 
plication, or ramification of species, it appears to me obvious that the primary 
condition required is that of preventing intercrossing with pai'cnt forms at the 
origin of each branch, whether the prevention be from the first absolute or only 
partial." 
Now it is evident that if we find in any species a tendency for individuals like 
one another in one or more characters to mate together, rather than with indi- 
viduals unlike themselves, we have at once a vera causa for the "divergence of 
individuals into varieties " by preventing intercrossing with parent forms. We 
need not even suppose that ixnions of unlike individuals are infertile provided the 
unions themselves do not occur or occur only rarely. The importance of homogamy 
has been so forcibly stated by Romanes* that I cannot do better than quote what 
he says. " To state the case in the most general terms we may say that if 
the other two basal principles are given in heredity and variability the whole 
theory of organic evolution becomes neither more nor less than a theory of 
homogamy — that is, a theory of the causes which lead to discriminate isolation, 
or the breeding of like with like to the exclusion of unlike. For the more we 
believe in heredity and variability as basal principles of organic evolution, the 
stronger must become our persuasion that discriminate breeding leads to divergence 
of type, while indiscriminate breeding leads to uniformity. This in fact is securely 
based on what we know from the experience supplied by artificial selection which 
consists in the intentional mating of like with like to the exclusion of unlike 
Only when assisted by some form of discriminate isolation which determines 
the exclusive breeding of like with like can heredity make in favour of change of 
type, or lead to what we understand by organic evolution." 
Now, although the importance of homogamy as a factor in evolution has been 
recognized almost universally, yet so far as I know no one except Pearson in the 
work on assortative mating in man, which has already been referred to (p. 214), has 
hitherto attempted to find out exactly how great a tendency for like to mate with 
like actually exists in a given species. We have had general reasoning in the place 
of direct quantitative evidence. In the present case it has been shown that in 
what may be considered the simplest prototype of the mating of individuals, 
which becomes in higher forms associated with sexual differentiation, namely in the 
conjugation of the Protozoa, there is a relatively very high degree of homogamy. 
Like pairs with like to more than double as close a degree as in the case of man 
where conscious choice must be supposed to operate to the greatest extent. We 
have then clearly all the necessary factors for divergent evolution. Let any 
variation appear among the pro-conjugant individuals (i.e. in the " conjugant 
type") of a race of Paramecium, and if any one of the characters in which the 
variation appears is correlated (to any degree) with any character directly selected 
in the homogamic pairing, we shall at once get the beginning of a divergent race 
or variety. If the explanation which has been given here (p. 267) of the method 
* Darwin and After Dartvin, Vol. iii. pp. G, 7. 
