ion at the expense of
their less fortunate brethren, and would leave a greater number of
offspring, of whom some possessed it in an even more marked degree than
themselves. And so it would go on. The process was a cumulative one. The
slightest variation in a favourable direction gave natural selection a
starting-point to work on. Through the continued action of natural
selection on each successive generation the useful variation was gradually
worked up, until at last it reached the magnitude of a specific {141}
distinction. Were it possible in such a case to have all the forms before
us, they would present the appearance of a long series imperceptibly
grading from one extreme to the other.

Upon this view are made two assumptions not unnatural in the absence of any
exact knowledge of the nature of heredity and variation. It was assumed, in
the first place that variation was a continuous process, and, second, that
any variation could be transmitted to the offspring. Both of these
assumptions have since been shown to be unjustified. Even before Mendel's
work became known Bateson had begun to call attention to the prevalence of
discontinuity in variation, and a few years later this was emphasised by
the Dutch botanist Hugo de Vries in his great work on _The Mutation
Theory_. The ferment of new ideas was already working in the solution, and
under the stimulus of Mendel's work they have rapidly crystallised out.
With the advent of heredity as a definite science we have been led to
revise our views as to the nature of variation, and consequently in some
respects as to the trend of evolution. Heritable variation has a definite
basis in the gamete, and it is to the gamete, therefore, not to the
individual, that we must look for the initiation of this process. Somewhere
or other in the course of their production is added or removed the factor
upon whose removal or addition the new variation owes its existence. The
new variation springs into being by a {142} sudden step, not by a process
of gradual and almost imperceptible augmentation. It is not continuous but
discontinuous, because it is based upon the presence or absence of some
definite factor or factors--upon discontinuity in the gametes from which it
sprang. Once formed, its continued existence is subject to the arbitrament
of natural selection. If of value in the struggle for existence natural
selection will decide that those who possess it shall have a better chance
of sur