The Making of Species 
both in first crosses and in hybrids, is an ex- 
tremely general result; but that it cannot, under 
our present state of knowledge, be considered as 
absolutely universal.” 
Similarly Wallace writes : ‘‘ 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.” 
Darwin resorted to much ingenious argument 
in his attempt to explain what he believed to 
be the almost universal sterility of hybrids, as 
opposed to mongrels or crosses between varieties. 
He pointed out that changed conditions tend to 
produce sterility, as is evidenced by the fact that 
many creatures refuse to breed in confinement, 
and believed that the crossing of distinct wild 
species produced a similar effect on the sexual 
organs. He expressed his belief that the early 
death of the embryos is a very frequent cause of 
sterility in first crosses. 
Wallace thus summarises Darwin's conclusions 
as to the cause of the sterility of hybrids: “The 
sterility or infertility of species with each other, 
whether manifested in the difficulty of obtaining 
first crosses between them or in the sterility of 
the hybrids thus obtained, is not a constant or 
necessary result of species difference, but is in- 
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