nvironmental conditions, and the
law of the Conditions of Existence to mean the law of adaptation to
environment. But that is not what Cuvier meant by the phrase: he
understood by it the principle of the co-ordination of the parts to form
the whole, the essential condition for the existence of any organism
whatsoever (see above, Chap. III., p. 34).

Of this thought there is in Darwin little trace, and that is why he did
not sufficiently appreciate the weight of the argument brought against
his theory that it did not account for the correlation of variations.

Darwin's conception of correlation was singularly incomplete. As
examples of correlation he advanced such trivial cases as the relation
between albinism, deafness and blue eyes in cats, or between the
tortoise-shell colour and the female sex. He used the word only in
connection with what he called "correlated variation," meaning by this
expression "that the whole organisation is so tied together during its
growth and development, that when slight variations in any one part
occur, and are accumulated through natural selection, other parts become
modified" (6th ed., p. 177). He took it for granted that the "correlated
variations" would be adapted to the original variation which was acted
upon by natural selection, and he saw no difficulty in the gradual
evolution of a complicated organ like the eye if only the steps were
small enough. "It has been objected," he writes, "that in order to
modify the eye and still preserve it as a perfect instrument, many
changes would have to be effected simultaneously, which, it is assumed,
could not be done through natural selection; but as I have attempted to
show in my work on the variation of domestic animals, it is not
necessary to suppose that the modifications were all simultaneous, if
they were extremely slight and gradual" (6th ed., p. 226).

In post-Darwinian speculation the difficulty of explaining correlated
variation by natural selection alone became more acutely realised, and
it was chiefly this difficulty that led Weismann to formulate his
hypothesis of germinal selection as a necessary supplement to the
general selection theory.

The change in the conception of correlation which Darwin's influence
brought about has been very clearly stated by E. von Hartmann,[359] from
whom the following is taken:--"While the correlation of parts in the
organism was before Darwin regarded exclusively from the standpoint of
morpholo