.
The classificatory work of {9} Linnaeus in the latter half of the
eighteenth century had given a definite significance to the word species,
and scientific men began to turn their attention to attempting to discover
how species were related to one another. And one obvious way of attacking
the problem was to cross different species together and see what happened.
This was largely done during the earlier half of the nineteenth century,
though such work was almost entirely confined to the botanists. Apart from
the fact that plants lend themselves to hybridisation work more readily
than animals, there was probably another reason why zoologists neglected
this form of investigation. The field of zoology is a wider one than that
of botany, presenting a far greater variety of type and structure. Partly
owing to their importance in the study of medicine, and partly owing to
their smaller numbers, the anatomy of the vegetable was far better known
than that of the animal kingdom. It is, therefore, not surprising that the
earlier part of the nineteenth century found the zoologists, under the
influence of Cuvier and his pupils, devoting their entire energies to
describing the anatomy of the new forms of animal life which careful search
at home and fresh voyages of discovery abroad were continually bringing to
light. During this period the zoologist had little inclination or
inducement to carry on those investigations in hybridisation which were
occupying the attention of some botanists. Nor did the efforts of the
botanists afford much {10} encouragement to such work, for in spite of the
labour devoted to these experiments, the results offered but a confused
tangle of facts, contributing in no apparent way to the solution of the
problem for which they had been undertaken. After half a century of
experimental hybridisation the determination of the relation of species and
varieties to one another seemed as remote as ever. Then in 1859 came the
_Origin of Species_, in which Darwin presented to the world a consistent
theory to account for the manner in which one species might have arisen
from another by a process of gradual evolution. Briefly put, that theory
was as follows: In any species of plant or animal the reproductive capacity
tends to outrun the available food supply, and the resulting competition
leads to an inevitable struggle for existence. Of all the individuals born,
only a portion, and that often a very small one, can survive