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have simultaneously varied.” But this is just what is
necessary, if there is any truth in the law of correlated
variation. It would be perfectly competent for any one
who did not believe in the existence of this principle to
argue that there is no such thing, and to maintain that it
is Natural Selection and not correlated variation which
produces co-ordinated structures. But to contend that the
law of correlated variation can exist in nature, and to
assume without proof that in certain cases it does not
come into action, seems most unreasonable. The law of
change in the organism is surely as permanent as the
organism itself, while the modifying cause from without
may vary. It is impossible to suppose that correlated
variation will be suppressed for unnumbered generations
in order that Natural Selection may have the field to itself.
But if the principle of correlated variation is always at
work, then every slight variation in one part will be
accompanied by slight variations in other parts, and the
choice will be between one co-ordinated structure and
another. In this case, we may ask what is the precise
selection-value of one of these co-ordinated structures
over another ? It is quite conceivable that the same
swiftness in flight, the same strength in effort, the same
capacity to overcome difficulty, may be manifested in a
given number of individuals, no two of which have exactly
the same co-ordination of parts.
But we will suppose, for the sake of argument, that indi
viduals are born with one part which is not correlated with
the rest of the structure ; that one stag has a heavier head
than is normal, and another a stronger neck, and another
sturdier legs, it is difficult to see what the selection-value of
such variants would be. Let us look at this matter from
the point of view of Natural Selection and sexual selec
tion ; for the two methods are not synonymous, although