Darwin was the first to take a broad survey of the whole range of
variations in the animal and vegetable kingdoms. His theory of Natural
Selection is based on the fact of variability. In order that this
foundation should be as strong as possible he collected all the facts,
scattered in the literature of his time, and tried to arrange them in a
scientific way. He succeeded in showing that variations may be grouped
along a line of almost continuous gradations, beginning with simple
differences in size and ending with monstrosities. He was struck by the
fact that, as a rule, the smaller the deviations, the more frequently they
appear, very abrupt breaks in characters being of rare occurrence.

Among these numerous degrees of variability Darwin was always on the look
out for those which might, with the greatest probability, be considered as
affording material for natural selection to act upon in the development of
new species. Neither of the extremes complied with his conceptions. He
often pointed out, that there are a good many small fluctuations, which in
this respect must be absolutely useless. On the other hand, he strongly
combated the belief, that great changes would be necessary to explain the
origin of species. Some authors had propounded the idea that highly
adapted organs, e.g. the wings of a bird, could not have been developed in
any other way than by a comparatively sudden modification of a well defined
and important kind. Such a conception would allow of great breaks or
discontinuity in the evolution of highly differentiated animals and plants,
shortening the time for the evolution of the whole organic kingdom and
getting over numerous difficulties inherent in the theory of slow and
gradual progress. It would, moreover, account for the genetic relation of
the larger groups of both animals and plants. It would, in a word,
undoubtedly afford an easy means of simplifying the problem of descent with
modification.