REV. JOHN GERARD, F.L.S., ON SPECIES AND THEIR ORIGIN. 141 
consideration. Of course the distinction to be drawn between 
“species” and “ varieties” constitutes a very great cruz, and we 
must on this matter defer in each case to the specialists. It is that 
to which Asa Gray referred when he said that he had had to do 
with the making and unmaking of so many species, that he had not 
much faith in the hard and fast definitions by’ which species were 
distinguished in handbooks of Botany ; like the man who “did not 
believe in ghosts, because he had seen too many of them.” (Natural 
Science and Religion: Scribner, New York.) That is an important 
confession. 
Each species is known by characters, which are established in each 
case by generalisations from those actually found in the individuals 
which compose the group ; and in every instance the generalisation 
is arrived at, as Mill would say, “by enumeration of instances.” 
There they are, however, transmissible in each species through many 
generations. Each individual is itself a “summation of powers,” 
including those which characterise the species and those which it 
shares in common with other species of the genus to which the 
species belongs ; so that we are thrown back upon the well-known 
necessity of proceeding in the definition of a species per genus et 
differentias (see Mill, Logie, B. i). 
The genus Equus, for example, contains not only the three 
modern species—caballus, asinus, and zebra—but others, as L. stenonis 
of the Italian region and £. sivalensis of the Indian region; both 
extinct since the Pliocene, yet with parts of skull, teeth, and lmb- 
bones sufficiently preserved to warrant the assignment of them to 
the genus Lyuwus and at the same time the differentiation of them 
structurally from the three modern species, with their manifold 
varieties. The descent of all these from the Miocene Anchitherium 
is pretty well established ; but many modifications are marked in the 
lines of descent, in which the influence of environment has played 
an important part. Judgments vary as to what constitute generic 
or specific differences. ‘Thus the form of ‘“ Horse ” now seen in the 
British Museum and labelled Hippidium neogvum was first described 
as a species of Eyuus; and the Equus caballus pregivalskiu, now 
accepted as the type of the original wild horse of Mongolia, was 
even thought by a very eminent naturalist to be a hybrid between 
the Tarpan and the Kiang. That however has been disproved 
since more individuals have been brought to England, and foals bred 
