the more important facts and
arguments adduced by each of the above writers, and how far they offer a
substitute for the action of natural selection; having done which, a
brief account will be given of the views of Dr. Aug. Weismann, whose
theory of heredity will, if established, strike at the very root of the
arguments of the first three of the writers above referred to.

_Mr. Herbert Spencer's Factors of Organic Evolution._

Mr. Spencer, while fully recognising the importance and wide range of
the principle of natural selection, thinks that sufficient weight has
not been given to the effects of use and disuse as a factor in
evolution, or to the direct action of the environment in determining or
modifying organic structures. As examples of the former class of
actions, he adduces the decreased size of the jaws in the civilised
races of mankind, the inheritance of nervous disease produced by
overwork, the great and inherited development of the udders in cows and
goats, and the shortened legs, jaws, and snout in improved races of
pigs--the two latter examples being quoted from Mr. Darwin,--and other
cases of like nature. As examples of the latter, Mr. Darwin is again
quoted as admitting that there are many cases in which the action of
similar conditions appears to have produced corresponding changes in
different species; and we have a very elaborate discussion of the direct
action of the medium in modifying the protoplasm of simple organisms, so
as to bring about the difference between the outer surface and the inner
part that characterises the cells or other units of which they are
formed.

Now, although this essay did little more than bring together facts which
had been already adduced by Mr. Darwin or by Mr. Spencer himself, and
lay stress upon their importance, its publication in a popular review
was immediately seized upon as "an avowed and definite declaration
against some of the leading ideas on which the Mechanical Philosophy
depends," and as being "fatal to the adequacy of the Mechanical
Philosophy as any explanation of organic evolution,"[198]--an expression
of opinion which would be repudiated by every Darwinian. For, even
admitting the interpretation which Mr. Spencer puts on the facts he
adduces, they are all included in the causes which Darwin himself
recognised as having acted in bringing about the infinitude of forms in
the organic world. In the concluding chapter of the _Origin of Species_
he says: "