460 
EECAPITULATION. 
Chap. XIV. 
It is^ no doubt, extremely difficult even to conjecture 
by wliat gradations many structures bave been per¬ 
fected, more especially amongst broken and failing 
groups of organic beings; but we see so many strange 
gradations in nature, that we ought to be extremely 
cautious in saying that any organ or instinct, or any 
whole being, could not have arrived at its present 
state by many graduated steps. There are, it must 
be admitted, cases of special difficulty on the theory of 
natural selection ; and one of the most curious of these 
is the existence of two or three defined castes of workers 
or sterile females in the same community of ants ; but 
I have attempted to show how this difficulty can be 
mastered. 
With respect to the almost universal sterility of 
species when first crossed, which forms so remarkable 
a contrast with the almost universal fertility of varieties 
when crossed, I must refer the reader to the recapitula¬ 
tion of the facts given at the end of the eighth chapter, 
which seem to me conclusively to show that this sterility 
is no more a special endowment than is the incapacity 
of two trees to be grafted together ; but that it is inci¬ 
dental on constitutional differences in the reproductive 
systems of the intercrossed species. We see the truth 
of this conclusion in the vast difference in the result, 
when the same two species are crossed reciprocally; 
that is, when one species is first used as the father and 
then as the mother. 
The fertility of varieties when intercrossed and of 
their mongrel offspring cannot be considered as uni¬ 
versal ; nor is their very general fertility surprising 
when we remember that it is not likely that either 
their constitutions or their reproductive systems should 
have been profoundly modified. Moreover, most of the 
varieties which have been experimentised on have been 
