to produce
offspring. According to Darwin's theory, the nature of the surviving
portion is not determined by chance alone. No two individuals of a species
are precisely alike, and among the variations that occur some enable their
possessors to cope more successfully with the competitive conditions under
which they exist. In comparison with their less favoured brethren they have
a better chance of surviving in the struggle for existence and consequently
of leaving offspring. The argument is completed by the further assumption
of a principle of heredity, in virtue of which offspring tend to {11}
resemble their parents more than other members of the species. Parents
possessing a favourable variation tend to transmit that variation to their
offspring, to some in greater, to others in less degree. Those possessing
it in greater degree will again have a better chance of survival, and will
transmit the favourable variation in even greater degree to some of their
offspring. A competitive struggle for existence working in combination with
certain principles of variation and heredity results in a slow and
continuous transformation of species through the operation of a process
which Darwin termed natural selection.

The coherence and simplicity of the theory, supported as it was by the
great array of facts which Darwin had patiently marshalled together,
rapidly gained the enthusiastic support of the great majority of
biologists. The problem of the relation of species at last appeared to be
solved, and for the next forty years zoologists and botanists were busily
engaged in classifying by the light of Darwin's theory the great masses of
anatomical facts which had already accumulated and in adding and
classifying fresh ones. The study of comparative anatomy and embryology
received a new stimulus, for with the acceptance of the theory of descent
with modification it became incumbent upon the biologist to demonstrate the
manner in which animals and plants differing widely in structure and
appearance could be conceivably related to one another. Thenceforward the
energies of both {12} botanists and zoologists have been devoted to the
construction of hypothetical pedigrees suggesting the various tracks of
evolution by which one group of animals or plants may have arisen from
another through a long continued process of natural selection. The result
of such work on the whole may be said to have shown that the diverse forms
under which livi