ENVIRONMENT AND EVOLUTION 37 



become so. Peculiarities of form or colour 

 are commonly associated with peculiarities of 

 locality or climate. All men who inhabit the 

 tropics are of dark complexion : brilliancy of 

 colour prevails amongst tropical birds and in- 

 sects : it is in temperate regions that birds 

 develop most tunefully their powers of song. We 

 can hardly believe that in these cases environ- 

 ment has not influenced the course of evolution. 

 Amongst those who deny that the influence of 

 environment can originate variations there are 

 some who admit that it may prepare for them 

 that the giraffe by striving to reach high branches 

 may endow its progeny with a disposition to 

 develop a longer neck. And it is indisputable 

 that a change of environment may influence 

 evolution by stimulating variation. It is under 

 the artificial conditions of domestication or 

 manuring that plants and animals " sport " most 

 freely. During the cycles that are covered by 

 the records of geology the surface of the earth 

 has undergone strange transformations. Land 

 has been submerged and re-elevated time after 

 time ; an ice cap has advanced over countries 

 that are now temperate, has retired, and has 

 advanced again ; mountain ranges have risen 

 and fallen like the waves of the sea. There have 

 been no lack of changes to stimulate variation, 

 and we may reasonably conclude that in former ages 

 they occurred more frequently than the present 

 condition of the earth might lead us to suppose. 

 Granting, however, that we can in some 

 measure explain the occurrence of variations, or 

 mutations, we are still confronted with a most 

 difficult problem the spread of these variations 

 from an individual to a race, the development of 

 a new species from single specimens which emerged 

 from their own kind bearing new peculiarities. 



