THEORIES OF ORGANIC EVOLUTION 



and a terrier have sprung from a common stock. So 

 have the Shetland pony, the thoroughbred, and the 

 draught-horse. In short, there is no domesticated ani- 

 mal that has not developed varieties deviating more or 

 less widely from the parent stock. Now, how has this 

 been accomplished? Why, clearly, by the preserva- 

 tion, through selective breeding, of seemingly accidental 

 variations. Thus one horseman, by constantly select- 

 ing animals that " chance" to have the right build and 

 stamina, finally develops a race of running-horses; 

 while another horseman, by selecting a different series 

 of progenitors, has developed a race of slow, heavy 

 draught animals. 



So far, so good ; the preservation of " accidental" va- 

 riations through selective breeding is plainly a means 

 by which races may be developed that are very differ- 

 ent from their original parent form. But this is under 

 man's supervision and direction. By what process 

 could such selection be brought about among creat- 

 ures in a state of nature? Here surely was a puzzle, 

 and one that must be solved before another step could 

 be taken in this direction. 



The key to the solution of this puzzle came into Dar- 

 win's mind through a chance reading of the famous 

 essay on " Population" which Thomas Robert Malthus 

 had published almost half a century before. This es- 

 say, expositing ideas by no means exclusively original 

 with Malthus, emphasizes the fact that organisms tend 

 to increase at a geometrical ratio through successive 

 generations, and hence would overpopulate the earth 

 if not somehow kept in check. Cogitating this thought, 

 Darwin gained a new insight into the processes of nat- 



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