THE THEORY OF LAMARCK 33 



We see, therefore, that the evidence in favour of the 

 existing species of animals and plants, having arisen 

 by a process of evolution, is of a most ample and con- 

 vincing kind. The theory of organic evolution is,- 

 however, incomplete until we have arrived at a true 

 account of the method or methods by which new 

 species arise from old ones. The earliest definite 

 explanation) as already stated, was that given by 

 Lamarck, and we may next proceed to consider the 

 Lamarckian theory of the origin of species. 



Earlier writers had already supposed that species 

 became modified through the action of the external 

 conditions to which they were exposed. Lamarck laid 

 special stress upon the observed facts that the organs 

 of individuals become increased and developed through 

 use, and that disuse is followed by a dwindling and 

 loss of the power of action. By the inherited effects 

 of use and disuse, and of modifications caused by ex- 

 ternal conditions, Lamarck supposed all evolution of 

 species to have come about. 



Reference has already been made to Lamarck's de- 

 scription of the method of origin of the characteristic 

 form of snakes, owing to the endeavours of the snakes' 

 ancestors to creep through narrow passages. Lamarck 

 was quite consistent inasmuch as he explained the 

 different types which have arisen among domesticated 

 species by the same theory as he applied to the origin 

 of species in a state of nature. Thus he supposed the 

 differences between race-horses and heavy cart-horses 

 to be the direct result of the different kinds of enforced 

 exercise to which the ancestors of these races were 



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