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yr EVOLUTION IN BIOLOGY 221 
rudimentary and apparently useless, in species, 
- the close allies of which possess well-developed 
and fuuctionally important homologous structures, 
are readily intelligible on the theory of evolution, 
while it is hard to conceive their ratson d’étre on 
any other hypothesis. However, a cautious rea- 
soner will probably rather explain such cases 
deductively from the doctrine of evolution than 
endeavour to support the doctrine of evolution by 
them. For it is almost impossible to prove that 
any structure, however rudimentary, is useless— 
that is to say, that it plays no part whatever in 
the economy; and, if it is in the slightest degree 
useful, there is no reason why, on the hypothesis 
_of direct creation, it should not have been created. 
Nevertheless, double-edged as is the argument 
from rudimentary organs, there is probably none 
which has produced a greater effect in promoting 
the general acceptance of the theory of evo- 
tution. 
6. The older advocates of evolution sought for the 
causes of the process exclusively in the influence of 
varying conditions, such as climate and station, or 
hybridisation, upon living forms. Even Treviranus 
has got no farther than this point. Lamarck in- 
troduced the conception of the action of an animal 
on itself as a factor in producing modification. 
Starting from the well-known fact that the 
habitual use of a limb tends to develop the muscles 
of the limb, and to produce a greater and greater 
