CHAP. VII 
OX THE INFERTILITY OF CROSSES 
153 
at all events had an origin quite distinct from that of varieties, 
this law could have no exceptions, because, if any two species 
had been found to be fertile when crossed and their hybrid 
offspring to be also fertile, this fact would have been held to 
prove them to be not species but varieties. On the other hand, 
if two varieties had been found to be infertile, or their mongrel 
offspring to be sterile, then it would have been said : These 
are not varieties but true species. Thus the old theory led 
to inevitable reasoning in a circle ; and what might be only a 
rather common fact was elevated into a law which had no 
exceptions. 
The elaborate and careful examination of the whole subject 
by Mr. Darwin, who has brought together a vast mass of 
evidence from the experience of agriculturists and horti¬ 
culturists, as well as from scientific experimenters, has demon¬ 
strated that there is no such fixed law in nature as was 
formerly supposed. He shows us that crosses between some 
varieties are infertile or even sterile, while crosses between 
some species are quite fertile ; and that there are besides a 
number of curious phenomena connected with the subject 
which render it impossible to believe that sterility is anything 
more than an incidental property of species, due to the 
extreme delicacy and susceptibility of the reproductive powers, 
and dependent on physiological causes we have not yet been 
able to trace. Nevertheless, the fact remains that most species 
which have hitherto been crossed produce sterile hybrids, as 
in the well-known case of the mule; while almost all domestic 
varieties, when crossed, produce offspring which are perfectly 
fertile among themselves. 1 will now endeavour to give such 
a sketch of the subject as may enable the reader to see some¬ 
thing of the complexity of the problem, referring him to Mr. 
Darwin’s works for fuller details. 
Extreme Susceptibility of the Reproductive Functions. 
One of the most interesting facts, as showing how sus¬ 
ceptible to changed conditions or to slight constitutional 
changes are the reproductive powers of animals, is the very 
general difficulty of getting those which are kept in confine¬ 
ment to breed ; and this is frequently the only bar to 
domesticating wild species. Thus, elephants, bears, foxes, 
