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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