Chap. YIII. 
FEKTILITY OF MONGRELS. 
269 
a remarkable fact; more especially when we reflect 
how many species there are, which, though resembling 
each other most closely, are utterly sterile when inter¬ 
crossed. Several considerations, however, render the 
fertility of domestic varieties less remarkable than at 
first appears. It can, in the first place, be clearly 
shown that mere external dissimilarity between two spe¬ 
cies does not determine their greater or lesser degree of 
sterility when crossed; and we may apply the same rule 
to domestic varieties. In the second place, some emi¬ 
nent naturalists believe that a long course of domesti¬ 
cation tends to eliminate sterility in the successive 
generations of hybrids which were at first only slightly 
sterile ; and if this be so, we surely ought not to expect 
to find sterility both appearing and disappearing under 
nearly the same conditions of life. Lastly, and this 
seems to me by far the most important consideration, 
new races of animals and plants are produced under 
domestication by man’s methodical and unconscious 
power of selection, for his own use and pleasure: he 
neither wishes to select, nor could select, slight differ¬ 
ences in the reproductive system, or other constitutional 
differences correlated with the reproductive system. 
He supplies his several varieties with the same food; 
treats them in nearly the same manner, and does not 
wish to alter their general habits of life. Nature acts 
uniformly and slowly during vast periods of time on the 
whole organisation, in any way which may be for each 
creature’s own good ; and thus she may, either directly, 
or more probably indirectly, through correlation, modify 
the reproductive system in the several descendants from 
any one species. Seeing this difference in the process 
of selection, as carried on by man and nature, we need 
not be surprised at some difference in the result. 
I have as yet spoken as if the varieties of the same 
