Chap. VIII. 
CAUSES OF STEKILITY. 
263 
We thus see, that although there is a clear and 
fundamental difference between the mere adhesion of 
grafted stocks, and the union of the male and female 
elements in the act of reproduction, yet that there is a 
rude degree of parallelism in the results of grafting and 
of crossing distinct species. And as we must look at 
the curious and complex laws governing the facility with 
which trees can be grafted on each other as incidental 
on unlmown differences in their vegetative systems, so 
I believe that the still more complex laws governing 
the facility of first crosses, are incidental on unknown 
differences, chiefly in their reproductive systems. 
These differences, in both cases, follow to a certain 
extent, as might have been expected, systematic affinity, 
by which every kind of resemblance and dissimilarity 
between organic beings is attempted to be expressed. 
The facts by no means seem to me to indicate that the 
greater or lesser difficulty of either grafting or crossing 
together various species has been a special endow¬ 
ment ; although in the case of crossing, the difficulty 
is as important for the endurance and stability of spe¬ 
cific forms, as in the case of grafting it is unimportant 
for their welfare. 
Causes of the Sterility of first Crosses and of Hybrids ,— 
We may now look a little closer at the probable causes 
of the sterility of first crosses and of hybrids. These 
two cases are fundamentally different, for, as just 
remarked, in the union of two pure species the male 
and female sexual elements are perfect, whereas in 
hybrids they are imperfect. Even in first crosses, the 
greater or lesser difficulty in effecting a union apparently 
depends on several distinct causes. There must some¬ 
times be a physical impossibility in the male element 
reaching the ovule, as would be the case with a plant 
