178 
Darwinism 
CUAP. 
physiological peculiarity. Thus, incipient species which were 
infertile together would have an advantage over other 
incipient species which were fertile, and, whenever the 
struggle for existence became severe, would prevail over them 
and take their place. Such infertility, being correlated with 
constitutional or structural differences, would probably, as 
already suggested, go on increasing as these differences 
increased; and thus, by the time the new species became 
fully differentiated from its parent form (or brother variety) 
the infertility might have become as well marked as we 
usually find it to be between distinct species. 
This discussion has led us to some conclusions of the greatest 
importance as bearing on the difficult problem of the cause of 
the sterility of the hybrids between distinct species. Accept¬ 
ing, as highly probable, the fact of variations in fertility 
occurring in correlation with variations in habits, colour, or 
structure, we see, that so long as such variations occurred only 
sporadically, and affected but a small proportion of the in¬ 
dividuals in any area, the infertility could not be increased by 
natural selection, but would tend to die out almost as fast as 
it was produced. If, however, it was so closely correlated 
with physical variations or diverse modes of life as to 
affect, even in a small degree, a considerable proportion of 
the individuals of the two forms in definite areas, it would 
be preserved by natural selection, and the portion of the 
varying species thus affected would increase at the expense of 
those portions which were more fertile when crossed. Each 
further variation towards infertility between the two forms 
would be again preserved, and thus the incipient infertility 
of the hybrid offspring might Ire increased till it became so 
great as almost to amount to sterility. Yet further, we have 
seen that if several competing species in the same area were 
being simultaneously modified, those between whose varieties 
infertility arose would have an advantage over those whose 
varieties remained fertile inter se, and would ultimately sup¬ 
plant them. 
The preceding argument, it will Ire seen, depends entirely 
upon the assumption that some amount of infertility char¬ 
acterises the distinct varieties which are in process of 
differentiation into species; and it may be objected that of 
