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it must preserve those who possess it, while those who 
do not possess it are destroyed. But we know that slight 
or even considerable modifications in the co-ordination 
of structure are perfectly compatible with fixity of species. 
I hold it, then, to be a very large assumption that, because 
a new attachment of the tendon to the bone may be ad 
vantageous, the existence of such a phenomenon presents 
a difficulty to which Mr. Darwin’s hypothesis gives us the 
key, and to which there is no other key. 
I have already dealt with the problem of mimicry, and 
need not here repeat the arguments already urged. 
Having established the necessity for the action of 
Natural Selection by ignoring his third factor of Organic 
Evolution, Mr. Spencer proceeds to limit the action of 
Natural Selection. It will not account for the dimin 
ished jaws of civilised man. It will not account for 
modifications which involve an elaborate co-ordination 
of structure. It will not come into action in those cases in 
which several members of a species may have various 
kinds of superiority over another. And on the ground of 
this inability of inherited use and disuse, and of Natural 
Selection, to produce all the phenomena connected with 
the transmutation of species, Mr. Spencer proceeds to argue 
that it is necessary to introduce the principle of the direct 
action of the medium. But here it may be observed that 
Mr. Spencer believes that the diminution of the jaw is 
brought about by use and disuse and the principle of 
correlation ; so that the necessity for a third factor is 
not established. If Natural Selection will not account 
for the complicated changes which some co-ordinated 
structures undergo, the inherited effects of use and disuse 
and the principle of correlation would give a sufficient ex 
planation, in some cases, at any rate ; and so far, again, 
the need for a third factor is not established. That