152 Evolution and, Adaptation 
“ At one time it appeared to me probable, as it has to 
others, that the sterility of first crosses and of hybrids might 
have been slowly acquired through the natural selection of 
slightly lessened degrees of fertility, which, like any other 
variation, spontaneously appeared in certain individuals of 
one variety when crossed with those of another variety. 
For it would clearly be advantageous to two varieties or in- 
cipient species, if they could be kept from blending, on 
the same principle that, when man is selecting at the same 
time two varieties, it is necessary that he should keep them 
. separate. 
“In considering the probability of natural selection having 
come into action, in rendering species mutually sterile, the 
greatest difficulty will be found to lie in the existence of 
many graduated steps from slightly lessened fertility to abso- 
lute sterility. It may be admitted that it would profit an 
incipient species, if it were rendered in some slight degree 
sterile when crossed with its parent form or with some other 
variety ; for thus fewer bastardized and deteriorated offspring 
would be produced to commingle their blood with the new 
species in process of formation. But he who will take the 
trouble to reflect on the steps by which this first degree of 
sterility could be increased through natural selection to that 
high degree which is common with so many species, and 
which is universal with species which have been differentiated 
to a generic or family rank, will find the subject extraordi- 
narily complex. After mature reflection it seems to me that 
this could not have been effected through natural selection. 
Take the case of any two species which, when crossed, pro- 
duced few and sterile offspring; now, what is there which 
could favor the survival of those individuals which happened 
to be endowed in a slightly higher degree with mutual infer- 
tility, and which thus approached by one small step toward 
absolute sterility? Yet an advance of this kind, if the theory 
of natural selection be brought to bear, must have inces- 
