_ THE ORIGIN OF SPECIES 27 
from the former point of view, a species is nothing 
‘more than a kind of animal or plant, which is \ 
distinctly definable from all others, by certain - 
/eonstant, and not merely sexual, morphological 
|peculiarities. Thus horses form a species, because 
|the group of animals to which that name is applied 
is distinguished from all others in the world by 
‘the following constantly associated characters. 
‘They have—i, A vertebral column; 2, Mammex; 
8, A placental embryo ; 4, Four legs; 5, A single 
well-developed toe in each foot provided with a 
hoof; 6, A bushy tail; and 7, Callosities on the 
inner sides of both the fore and the hind legs. 
The asses, again, form a distinct species, because, 
with the same characters, as far as the fifth in the 
above list, all asses have tufted tails, and have 
callosities only on the inner side of the fore-legs. 
If animals were discovered having the general 
characters of the horse, but sometimes with cal- 
losities only on the fore-legs, and more or less 
tufted tails ; or animals having the general char- 
acters of the ass, but with more or less bushy 
tails, and sometimes with callosities on both pairs 
of legs, besides being intermediate in other re- 
spects—the two species would have to be merged 
into one. They could no longer be regarded as 
morphologically distinct species, for they would 
not be distinctly definable one'from the other. 
However bare and simple this definition of 
species may appear to be, we confidently appeal to 
