rn THE ORIGIN OF SPECIES 45 
‘but the offspring of such crossed races are perfectly 
‘fertile with one another. Thus, the spaniel and 
ithe greyhound, the dray-horse and the Arab, the 
| pouter and the tumbler, breed together with perfect 
freedom, and their mongrels, if matched with other 
mongrels of the same kind, are equally fertile. 
On the other hand, there can be no doubt that 
the individuals of many natural species are either 
absolutely infertile if crossed with individuals of 
other species, or, if they give rise to hybrid 
offspring, the hybrids so produced are infertile 
when paired together. The horse and the ass, 
for instance, if so crossed, give rise to the mule, 
and there is no certain evidence of offspring ever 
having been produced by a male and female 
mule. The unions of the rock-pigeon and the 
ring-pigeon appear to be equally barren of result. 
Here, then, says the physiologist, we have a means 
of distinguishing any two true species from any 
_ two varieties. Ifa male and a female, selected 
“from each group, produce offspring, and that off- 
spring is fertile with others produced in the same 
" way, the groups are races and not species. If, on 
the other hand, no result ensues, or if the offspring 
are infertile with others produced in the same 
way, they are true physiological species. The 
test would be an admirable one, if, in the first 
place, it were always practicable to apply it, and 
if, in the second, it always yielded results suscep- 
tible of a definite interpretation. Unfortunately, 
