1897] REPRODUCTIVE DIVERGENCE 189 
viduals of greater vigour and fitness than the parents. As far as I 
am aware, there is no evidence to show that this greater vigour is 
the result of the differences of morphological form, but rather that 
it is due to the individuals being descended from different stocks, 
whereby the evil effects of in-and-in breeding are avoided, or to 
being exposed to differences of environmental conditions, whereby 
they may perhaps be rendered physiologically unlike individuals 
to a slight degree, rather than morphologically unlike. That mere 
exposure to differences of environmental conditions may be sufficient 
to give rise to a vigorous race even when this is propagated by the 
closest in-and-in breeding, is shown by the case of the rabbits on 
the Island of Porto Santo, all of which are descended from a single 
pregnant individual. 
Another objection which might be raised is, that in the case of 
both plants and animals it has frequently been found that varieties 
showing considerable differences of external form are perfectly 
fertile inter se. Even if this is the case, it is no argument against 
the theory of reproductive divergence, for it was specially men- 
tioned that this is not supposed to be invariably in operation when 
a species is in course of splitting up into varieties. At the same 
time, it may reasonably be doubted whether this statement as to 
the perfect fertility of varieties is a fact, because a very slight 
degree of sterility would easily escape notice unless extensive series 
of breeding experiments were made, and careful records kept. 
H. M. VERNON. 
