ia) rae oo gyeme were te ie 
48 THE ORIGIN OF SPECIES 
between the same two species. It is not always equal in degre 
in a first cross, and in the hybrid produced from this cross. 
‘*In the same manner as in grafting trees, the capacity of 
one species or .variety to take on another is incidental on 
generally unknown differences in their vegetative systems ; so 
in crossing, the greater or less facility of one species to unite 
with another is incidental on unknown differences in their 
reproductive systems. There is no more reason to think that 
species have been specially endowed with various degrees of — 
sterility to prevent them crossing and breeding in Nature, thar 
to think that trees have been specially endowed with various — 
and somewhat analogous degrees of difficulty in being grafted — 
together, in order to prevent them becoming inarched in our 
forests. } 
** The sterility of first crosses between pure species, which 
have their reproductive systems perfect, seems to depend on 
several circumstances ; in some cases largely on the early death of — 
the embryo. The sterility of hybrids which have their repro- 
ductive systems imperfect, and which have had this system 
and their whole organisation disturbed by being compounded 
of two distinct species, seems closely allied to that sterility 
which so frequently affects pure species when their natural con- 
ditions of life have been disturbed. This view is supported by 
a parallelism of another kind: namely, that the crossing of 
forms, only slightly different, is favourable to the vigour and 
fertility of the offspring ; and that slight changes in the con-— 
ditions of life are apparently favourable to the vigour and 
fertility of all organic beings. It is not surprising that the 
degree of difficulty in uniting two species, and the degree of 
sterility of their hybrid offspring, should generally correspond, 
though due to distinct causes ; for both depend on the amount 
of difference of some kind between the species which are crossed. — 
Nor is it surprising that the facility of effecting a first cross, 
the fertility of hybrids produced from it, and the capacity of — 
being grafted together—though this latter capacity evidently 
depends on widely different cireumstances—should all run to a 
certain extent parallel with the systematic affinity of the forms — 
which are subjected to experiment; for systematic affinity - 
