5 
The value of species has been admirably expressed by 
Professor Huxley:—“Thus horses form a species, because 
the group of animals to which that name is applied is dis¬ 
tinguished from all others in the world by the following 
constantly associated characters:—They have—i. A verte¬ 
bral column ; 2. Mammae; 3. A placental embryo ; 4. Four 
legs; 5. A single well-developed toe on a foot provided with 
a hoof; 6. A bushy tail; and 7. Callosities on the inner side 
of both the fore and hind legs. The asses, again, form a dis¬ 
tinct species, because, with the same characters, as far as the 
fifth in the above list, all asses have tufted tails, and have cal¬ 
losities only on the inner side of the forelegs. If animals 
were discovered having all the general characters of the 
horse, but sometimes with callosities only on the forelegs, 
and more or less tufted tails, or animals having the general 
characters of the ass, but with more or less bushy tails, and 
sometimes with callosities on both pairs of legs, besides being 
intermediate in other respects, 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 dis¬ 
tinctly definable one from the other .”—Westminster Review , 
April, i860. 
