FIFTY YEARS OF DARWINISM 47 



while he interpreted the poverty of Greenland 

 flora and the reappearance of north temperate 

 species in the southern part of South America as 

 results of the same cause. Almost as soon as the 

 facts were before him in WoUaston's memoirs, 

 Darwin had interpreted the number of wingless 

 beetles in oceanic islands as due to the special 

 dangers of flight. He anticipated H. W. Bates' 

 hypothesis of mimicry, but drove it from his mind 

 because he did not feel confident about the geo- 

 graphical coincidence of model and mimic. Long 

 before the Origin appeared Darwin had thought 

 over and rejected the idea that the same species 

 could have more than a single origin or could 

 arise independently in two different countries — 

 a hypothesis very popular in later years, but, I 

 believe, now entirely abandoned. 



I should wish to advance one consideration be- 

 fore concluding this section of my address. Cer- 

 tain writers on mutation seem to hold the view that 

 Natural Selection alone prevents large variations 

 from often holding the field and leading to great 

 and rapid changes of form. Such a view is not 

 supported by the history of species which inhabit 

 situations comparatively sheltered from the 

 struggle, such as fresh water, caves, certain 

 islands, or the depth of the ocean. Organisms 

 in these places tend to preserve their ancestral 

 structure more persistently than in the crowded 

 areas where Natural Selection holds more potent 

 sway. 



