186 NATURAL SCIENCE [September 
characteristics, unless of course other causes are at work counter- 
acting its influence. Probably in the majority of cases, in most 
stable species in fact, there is no such variability of fertility between 
slightly differing individuals, and hence there is, from this cause, no 
tendency to the formation of more or less distinct varieties. Very 
probably, however, there is a latent possibility of such a variability of 
reproductive power arising in almost any species, when for instance 
some of its members are exposed to fresh environmental conditions, 
in consequence of migration or change of climate. If this is so, 
then a species will tend to split up into varieties just at the most 
opportune moment, the varieties thus formed becoming by the action 
of Natural Selection gradually more and more adapted to their sur- 
roundings, and so fresh species produced. That change of environ- 
mental conditions has a very great influence on the reproductive 
powers of both animals and plants is a well-known fact, and one on 
which Darwin has collected much valuable evidence.! | 
It now remains to be demonstrated how the theory of reproduc- 
tive divergence can successfully account for some of the chief objec- 
tions which have been brought against the theory of Natural Selec- 
tion, objections indeed which have been of considerable weight in 
deciding many scientists against the doctrine of the all-sufficiency of 
Natural Selection as the cause of Evolution. 
The fact of the very general infertility of crosses between species 
and their hybrid offspring, coupled with that of the fertility of 
crosses between varieties, and of their mongrel offspring, was recog- 
nised by Darwin as a formidable objection. Though this distinction 
between species and varieties is now recognised as not of such uni- 
versality as it was formerly believed to be, yet it is still admitted to 
be a difficulty hitherto by no means adequately accounted for. The 
theory of reproductive divergence offers a most satisfactory and con- 
vincing explanation. Thus according to it, as we have seen, varieties 
and ultimately new species have, in many cases at least, been formed 
by the operation of a slight and accumulating sterility between 
unlike individuals, whereby two or more groups of individuals be- 
come more and more segregated, and so capable of undergoing inde- 
pendent variation. This divergence of species takes place quite 
independently of Natural Selection, but this principle can always be 
exerting its action at the same time, whereby the new or modified 
characteristics produced can, if useful to the species, be accumulated 
and rendered better adapted to the environmental conditions. 
Whether the very general sterility of crosses between species is 
due originally in most or in all cases to reproductive divergence, or 
whether it came into operation but seldom, it is not as yet possible 
to say. If extended series of experiments show that it is in fairly 
“Variation of Animals and Plants under Domestication,” vol. ii., pp. 130-149. 
