274 THE THEORIES OF EVOLUTION 



certain determined lines of development (orthogen- 

 esis), not for certain passive but complex cases of 

 adaptation (mimicry). 



As far as these and many other puzzling problems 

 are concerned, we can only follow the path opened by 

 the discoveries of modern scientists. Certain complex 

 questions, such as mimicry, seem a little less obscure 

 when studied in the light of the Lamarckian theories. 

 For instance, the protective white colouring of animals 

 inhabiting the polar regions, which had long been at- 

 tributed to the action of natural selection, results 

 merely, according to Metchnikoff 's observations, from 

 the direct influence of cold, which in the course of 

 experiments caused hair and feathers to turn white. 

 It appears probable, in view of the increasing fre- 

 quency of such experiments, that many other prob- 

 lems will be solved in similar manner. 



