THE THEORY OF LAMARCK 37 



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 

 respectively subjected. Similarly, all the different 

 breeds of dogs were supposed to have arisen owing to 

 the different habits which the various successors of the 

 first domesticated dogs acquired, small changes being 

 accumulated by inheritance in each successive genera- 

 tion. 



Turning now to species in a state of nature, the case 

 of the giraffe is one of those most often quoted. 

 Lamarck supposed a comparatively . short - necked 

 ancestor of the giraffes to have taken up the habit of 

 browsing upon the leaves of trees, owing to the diffi- 

 culty of obtaining other food in an arid region. In 

 order to obtain their new food the animals were obliged 

 to be continually stretching upward, and the effort to 

 elongate their necks was attended with some small 



