Chap. VIII. 
CAUSES OF STEEILITY. 
265 
very different. I have more than once alluded to a 
large body of facts, which I have collected, showing 
that when animals and plants are removed from their 
natiu’al conditions, they are extremely liable to have their 
reproductive systems seriously affected. This, in fact, is 
the great bar to the domestication of animals. Between 
the sterility thus superinduced and that of hybrids, there 
are many points of similarity. In both cases the sterility 
is independent of general health, and is often accom¬ 
panied by excess of size or great luxuriance. In both 
cases, the sterility occurs in various degrees; in both, 
the male element is the most liable to be affected; but 
sometimes the female more than the male. In both, 
the tendency goes to a certain extent with systematic 
affinity, for whole groups of animals and plants are ren¬ 
dered impotent by the same unnatural conditions ; and 
whole groups of species tend to produce sterile hybrids. 
On the other hand, one species in a group will some¬ 
times resist great changes of conditions with unimpaired 
fertility; and certain species in a group will produce 
unusually fertile hybrids. No one can tell, till he tries, 
whether any particular animal will breed under confine¬ 
ment or any exotic plant seed freely under culture ; nor 
can he tell, till he tries, whether any two species of a 
genus will produce more or less sterile hybrids. Lastly, 
when organic beings are placed during several genera¬ 
tions under conditions not natural to them, they are 
extremely liable to vary, which is due, as I believe, to 
their reproductive systems having been specially affected, 
though in a lesser degree than when sterility ensues. So 
it is with hybrids, for hybrids in successive generations 
are eminently Liable to vary, as every experimentalist 
has observed. 
Thus we see that when organic beings are placed 
under new and unnatural conditions, and when hybrids 
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