hem to
a physiological cycle which he forthwith {222} agreed to ignore
altogether,[2] he confined his attention to the causes of preservation,
and under the names of natural selection and sexual selection studied
them exclusively as functions of the cycle of the environment.

Pre-Darwinian philosophers had also tried to establish the doctrine of
descent with modification; but they all committed the blunder of
clumping the two cycles of causation into one. What preserves an
animal with his peculiarity, if it be a useful one, they saw to be the
nature of the environment to which the peculiarity was adjusted. The
giraffe with his peculiar neck is preserved by the fact that there are
in his environment tall trees whose leaves he can digest. But these
philosophers went further, and said that the presence of the trees not
only maintained an animal with a long neck to browse upon their
branches, but also produced him. They _made_ his neck long by the
constant striving they aroused in him to reach up to them. The
environment, in short, was supposed by these writers to mould the
animal by a kind of direct pressure, very much as a seal presses the
wax into harmony with itself. Numerous instances were given of the way
in which this goes on under our eyes. The exercise of the forge makes
the right arm strong, the palm grows callous to the oar, the mountain
air distends the chest, the chased fox grows cunning and the chased
bird shy, the arctic cold stimulates the animal combustion, and so
forth. Now these changes, of which many more examples might be
adduced, are {223} at present distinguished by the special name of
_adaptive_ changes. Their peculiarity is that that very feature in the
environment to which the animal's nature grows adjusted, itself
produces the adjustment. The 'inner relation,' to use Mr. Spencer's
phrase, 'corresponds' with its own efficient cause.

Darwin's first achievement was to show the utter insignificance in
amount of these changes produced by direct adaptation, the immensely
greater mass of changes being produced by internal molecular accidents,
of which we know nothing. His next achievement was to define the true
problem with which we have to deal when we study the effects of the
visible environment on the animal. That problem is simply this; Is the
environment more likely to _preserve or to destroy him_, on account of
this or that peculiarity with which he may be born? In giving the name
o