PME eel YOR SPE CEES. 67 
any rate it is quite certain that the hybrids are often 
absolutely infertile one with another. 
“Here is a feature, then, great or small as it may be, 
which distinguishes natural species of animals. Can 
we find any approximation to this in the different races 
known to be produced by selective breeding from a 
common stock? Up to the present time the answer to 
that question is absolutely a negative one. ‘As far as 
we know at present, there is nothing approximating to 
this check. “In crossing the breeds, between the fantail 
and the pouter, the carrier and the tumbler, or any other 
variety or race you may name—so far as we know at 
present—there is no difficulty in breeding together the 
mongrels.” However, he continues, as soon as you re- 
move the conditions which produced the new variety,— 
as when you permit pigeons to mate promiscuously,—no 
matter how different the varieties may have been, you 
will have, in a few generations of pigeons, the same blue 
rock pigeon with the black bars across the wings. No 
new species has originated. All varieties, in a free state, 
revert to type. “This,” says Huxley, “is certainly a 
very remarkable circumstance.” 
Fairhurst points out the difficulties in which the evo- 
lutionist becomes involved through the fixity of species. 
He writes: “It is well known that as a rule distinct 
species will not cross, and that if they do cross the off- 
spring are not fertile. On the other hand, it is true 
that all varieties of a species readily cross, producing 
fertile offspring. This has commonly been regarded as 
a well-defined distinction between varieties and species. 
If the varieties of pigeons which are so different from 
each other did not freely cross, and if the mongrel off- 
spring were not fertile, Darwin’s argument as to the 
production of new species under domestication would be 
complete. The fact is, we do not know of the origin of 
