struck by the innumerable cases of adaptation, as, for instance, that of
the woodpeckers and tree-frogs to climbing, or the hooks and feather-like
appendages of seeds, which aid in the distribution of plants, and he said
to himself that an explanation of adaptations was the first thing to be
sought for in attempting to formulate a theory of evolution.

But since adaptations point to CHANGES which have been undergone by the
ancestral forms of existing species, it is necessary, first of all, to
inquire how far species in general are VARIABLE. Thus Darwin's attention
was directed in the first place to the phenomenon of variability, and the
use man has made of this, from very early times, in the breeding of his
domesticated animals and cultivated plants. He inquired carefully how
breeders set to work, when they wished to modify the structure and
appearance of a species to their own ends, and it was soon clear to him
that SELECTION FOR BREEDING PURPOSES played the chief part.

But how was it possible that such processes should occur in free nature?
Who is here the breeder, making the selection, choosing out one individual
to bring forth offspring and rejecting others? That was the problem that
for a long time remained a riddle to him.

Darwin himself relates how illumination suddenly came to him. He had been
reading, for his own pleasure, Malthus' book on Population, and, as he had
long known from numerous observations, that every species gives rise to
many more descendants than ever attain to maturity, and that, therefore,
the greater number of the descendants of a species perish without
reproducing, the idea came to him that the decision as to which member of a
species was to perish, and which was to attain to maturity and reproduction
might not be a matter of chance, but might be determined by the
constitution of the individuals themselves, according as they were more or