which is significant for his present purpose; and second,
the large knowledge of the subject which will enable him to follow it
into other cases in which it occurs with different circumstances, or, in
other words, to follow a similarity through diverse cases. Darwin's
great achievement in establishing the principle of evolution lay first
in the scientific sagacity which flashed home on him, after years of
patient study, that the one common fact in all the multitude of plants
and animals is that in the struggle for existence by which all living
beings persist, those who are best fitted to their circumstances
survive; and second, in his rich knowledge of the world of nature, which
made it possible for him to follow out this characteristic in all kinds
of plants and animals, and so to reach the general law. But whether it
be so world-sweeping a conclusion as his, or my conclusion that my dog
has killed a hen, the process is the same: analysis or breaking up of
the complex fact, and following out the consequences or implications of
some selected part of it into other cases.

All reasoning thus reduces itself in the end to a process of passing
from like to like: we notice that the present case is like other cases
which we already know: then, since these cases have always in the past
been accompanied by certain circumstances or consequences, we believe
that the present case will also show these same circumstances or
consequences. Whenever my dog has killed when the cases have been
similar in the blood and feathers on his mouth; in this case he has
blood and feathers on his mouth; therefore he must have killed a hen.
Individual plants and animals survive which are fitted to their
environment by special characteristics, and those which are not so
fitted die; species of plants and animals, as well as individuals, show
special adaptation to their environment; therefore species have survived
through the same process of natural selection.

It follows that reasoning, whether it results in a general law or in
concrete judgment, depends on the assumption that nature--and in nature
we mean here the whole universe as we know it is uniform; that there are
ties between facts which make it possible for us to be certain that if a
given fact occurs, then another fact always occurs with it as an effect,
or as a cause, or connected with it in some other manner. Without this
certainty of the uniformity of things there would be no reasoning, an