312 EVOLUTION IN BIOLOGY. 



5. The innumerable cases of structures, which are 

 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 reasoner 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 evolution. 



6. The older advocates of evolution sought for the 

 causes of the process exclusively in the influence of vary- 

 ing conditions, such as climate and station, or hybridi- 

 sation, upon living forms. Even Treviranus has got no 

 farther than this point. Lamarck introduced the con- 

 ception 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 facility in using it, he made the general assump- 

 tion that the effort of an animal to exert an organ in a 



