THE DES TE UCTIVE A GENCIES OF NA THEE. 1 59 



in other words, only so long as it can maintain 

 its ground against enemies and competitors. 

 The constant interaction of one species upon 

 another, combined with the gradual but ever 

 varying influence of climatic changes, and 

 altered conditions of life, constantly maintain 

 every animal and plant which does not succumb 

 to them, at the exact point of training required 

 by the exigencies of its existence. The law of 

 physical nature is let those live that can, and 

 produce a stronger race : and let those die 

 that must, and yield their place to others. 



Human agency is at present the most obvious 

 cause of the extinction of species, and ne- 

 cessarily so, although its effect has been 

 exaggerated, for it is not dominant, but local 

 or waning species which are exterminated by 

 man. It was perhaps not human agency 

 which ultimately restricted the range of Rhytina 

 Stelleri to two small islands in Behring's Straits, 

 where it was extirpated by man within a 

 few years after the discovery of its haunts. 

 In the early geological ages, the destruction of 

 species of course arose from so-called " natural 

 causes " alone, but man is now so dominant 

 that all other creatures hold a completely 



