ted the modification to the change of
circumstances.

The two volumes comprising the "Origin of Species" constitute, as the
author said, one long argument. It is, of course, impossible in the
space at our command to recapitulate in detail even the leading facts
and inferences which are brought forward to prove that species have been
modified during a long course of descent. We must confine ourselves to a
succinct statement of the author's general conclusions. What he
undertakes to prove is that the modification of species during a long
course of descent has been effected chiefly through the natural
selection of numerous successive slight favorable variations, aided in
an important manner by the inherited effects of the use and disuse of
parts; and in an unimportant manner,--that is, in relation to adaptive
structures, whether past or present, by the direct action of external
conditions, and by variations which seem to us, in our ignorance, to
arise spontaneously. It should be observed that Darwin does not
attribute the modification exclusively to natural selection. What he
asserts is: "I am convinced that natural selection has been the main,
but not the exclusive, means of modification." He submits that a false
theory would hardly explain in so satisfactory a manner as does the
theory of natural selection the several large classes of facts
marshalled in the two volumes now under review. If it be objected that
this is an unsafe method of arguing, Darwin rejoins that it is a method
usual in judging of the common events of life, and has often been used
by the greatest natural philosophers. The undulatory theory of light,
for instance, has thus been arrived at; and the belief in the revolution
of the earth on its own axis was, until lately, supported by scarcely
any direct evidence. It is no valid objection to the Darwinian theory of
the origin of species that science as yet throws no light on the far
higher problem of the essence or origin of life. Neither has any one
explained what is the essence of the attraction of gravity, though
nobody now objects to following out the results consequent on this
unknown element of attraction.

Why, it may be asked, did nearly all the most eminent naturalists and
geologists until recently decline to believe in the mutability of
species? Darwin replies that the belief that species were immutable
productions was almost unavoidable as long as the history of the world
was thought to be of