Chap. VIII. 
SUMMAEY. 
,277 
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 reproductive systems im¬ 
perfect, and which have had this system and their whole 
organisation disturbed by being compounded of two dis¬ 
tinct species, seems closely allied to that sterility which 
so frequently affects pure species, when their natural 
conditions 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 their offspring ; 
and that slight changes in the conditions of life are ap¬ 
parently 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 cor¬ 
respond, 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 the 
hybrids produced from it, and the capacity of being 
grafted together—though this latter capacity evidently 
depends on widely different circumstances—should all 
run, to a certain extent, parallel with the systematic 
affinity of the forms which are subjected to experiment; 
for systematic affinity attempts to express all kinds of 
resemblance between all species. 
First crosses between forms known to be varieties, or 
sufficiently alike to be considered as varieties, and their 
mongrel offspring, are very generally, but not quite uni- 
