TI EVOLUTION IN BIOLOGY 221 



rudimentary and apparently useless, in species, 

 the close allies of which possess well-developed 

 and functionally important homologous structures, 

 are readily intelligible on the theory of evolution, 

 while it is hard to conceive their raison d'etre 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- 

 lution. 



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 



