, nor at the time had he formulated any hypothesis of his own.
His mind was open and receptive; he was eager only for facts which might
lead him to an understanding of a problem which seemed utterly obscure.
It was something to feel sure that species have varied; but how have
such variations been brought about?

It was not long before Darwin found a clew which he thought might
lead to the answer he sought. In casting about for facts he had soon
discovered that the most available field for observation lay among
domesticated animals, whose numerous variations within specific lines
are familiar to every one. Thus under domestication creatures so
tangibly different as a mastiff and a terrier have sprung from a
common stock. So have the Shetland pony, the thoroughbred, and the
draught-horse. In short, there is no domesticated animal that has not
developed varieties deviating more or less widely from the parent stock.
Now, how has this been accomplished? Why, clearly, by the preservation,
through selective breeding, of seemingly accidental variations. Thus
one horseman, by constantly selecting animals that "chance" to have
the right build and stamina, finally develops a race of running-horses;
while another horseman, by selecting a different series of progenitors,
has developed a race of slow, heavy draught animals.

So far, so good; the preservation of "accidental" variations through
selective breeding is plainly a means by which races may be developed
that are very different from their original parent form. But this
is under man's supervision and direction. By what process could such
selection be brought about among creatures in a state of nature? Here
surely was a puzzle, and one that must be solved before another step
could be taken in this direction.

The key to the solution of this puzzle came into Darwin's mind through
a chance reading of the famous essay on "Population" which Thomas
Robert Malthus had published almost half a century before. This
essay, expositing ideas by no means exclusively original with Malthus,
emphasizes the fact that organisms tend to increase at a geometrical
ratio through successive generations, and hence would overpopulate the
earth if not somehow kept in check. Cogitating this thought, Darwin
gained a new insight into the processes of nature. He saw that in virtue
of this tendency of each race of beings to overpopulate the earth,
the entire organic world, animal and vegetable, must be in a