Darwin's Artificial and Natural Selection 151 
crosses between the same two species, the male sexual ele- 
ment of the one will often freely act on the female sexual 
element of the other, but not in a reversed direction.” 
Does Darwin give here a satisfactory answer to the diffi- 
culty that he started out to explain away? On the whole, 
the reader will admit, I think, that he has fairly met the sit- 
uation, in so far as he has shown that there is no absolute 
line of demarcation between the power of intercrossing of 
varieties and races, and of species. It is also extremely im- 
portant to have found that the difficulties increase, so to speak, 
even beyond the limtts of the species ; since species, belonging 
to different genera, are as a rule more difficult to intercross 
than when they belong to the same genus. The further 
question, as to whether there are differences in respect to the 
power of intercrossing between different kinds of varieties, 
such as those dependent on selection of fluctuating varia- 
tions, of local conditions, of mutations, etc., is far from being 
settled at the present time. 
That this property of species is useful to them, in the some- 
what unusual sense that it keeps them from freely mingling 
with other species, is true; but, as has been said, this would be 
a rather peculiar kind of adaptation. If, however, it be 
claimed that this property is useful to species, as Darwin 
himself claims, then, as he also points out, it is a useful 
acquirement that cannot have arisen through natural selec- 
tion. It is not difficult to show why this must be so. If two 
varieties were to some extent at the start less fertile, zzer se, 
than with their own kind, the only way in which they could 
become more infertile through selection would be by selecting 
those individuals in each generation that are still more infer- 
tile, but the forms of this sort would, ex hypothese, become 
less numerous than the descendants of each species itself, 
which would, therefore, supplant the less fertile ones. 
Darwin’s own statement in regard to this point is as fol- 
lows :— 
