THE CENTURY'S PROGRESS IN BIOLOGY 



most available field for observation lay among domesti- 

 cated animals, whose numerous variations within specific 

 lines are familiar to every one. Thus under domestica- 

 tion creatures so tangibly different as a mastiff 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 animal that 

 has not developed varieties deviating more or less wide- 

 ly from the parent stock. Now how has this been ac- 

 complished ? Why, clearly, by the preservation, through 

 selective breeding, of seemingly accidental variations. 

 Thus one horseman, by constantly selecting 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 different 

 from their original parent form. But this is under 

 man's supervision and direction. By what process could 

 such selection be brought about among creatures 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 Kobert Malthus 

 had published almost half a century before. This essay, 

 expositing ideas by no means exclusively original with 

 Malthus, emphasizes the fact that organisms tend to 

 increase at a geometrical ratio through successive gen- 

 erations, and hence would overpopulate the earth if not 

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