222 
THE DESCENT OF MAN. 
Part D 
there is in hybrids between lessened fertility and vitality t 
other analogous cases could be added. 
Even if it should hereafter be proved that all the 
races of men were perfectly fertile together, he who w 7 as 
inclined from other reasons to rank them as distinct 
species, might with justice argue that fertility and 
sterility are not safe criterions of specific distinctness. 
We know that these qualities are easily affected by 
changed conditions of life or by close inter-breedings 
and that they are governed by highly complex laws, for 
instance that of the unequal fertility of reciprocal crosses 
between the same two species. With forms which must 
be ranked as undoubted species, a perfect series exists 
from those which are absolutely sterile when crossed,, 
to those which are almost or quite fertile. The degrees 
of sterility do not coincide strictly with the degrees of 
difference in external structure or habits of life. Man 
in many respects may be compared with those animals 
which have long been domesticated, and a large body 
of evidence can be advanced in favour of the Pallasian 
doctrine 13 that domestication tends to eliminate the 
13 4 Tlie Variation of Animals and Plants under Domestication,’ vol. ii. 
p. 109. I may here remind the reader that the sterility of species 
when crossed is not a specially-acquired quality ; but, like the inca- 
pacity of certain trees to be grafted together, is incidental on other 
acquired differences. The nature of these differences is unknown, but 
they relate more especially to the reproductive system, and much les& 
to external structure or to ordinary differences in constitution. One 
important element in the sterility of crossed species apparently lies in 
one or both having been long habituated to fixed conditions ; for we 
know that changed conditions have a special influence on the repro- 
ductive system, and we have good reason to believe (as before re- 
marked) that the fluctuating conditions of domestication tend to elimi- 
nate that sterility which is so general with species in a natural state 
when crossed. It has elsewhere been shewn by me (ibid. vol. ii. p. 185, 
and 4 Origin of Species,’ 5th edit. p. 317) that the sterility of crossed 
species has not been acquired through natural selection : we can see that 
when two forms have already been rendered very sterile, it is scarcely 
