ix] OF EVOLUTION 107 



Fifteen months after this ' systematic enquiry' 

 began, Darwin happened to read the celebrated work 

 of Malthus On Population, for amusement, and this 

 served as a spark falling on a long prepared train 

 of thought. The idea that as animals and plants 

 multiply in geometrical progression, while the 

 supplies of food and space to be occupied remain 

 nearly constant, and that this must lead to a ' struggle 

 for existence ' of the most desperate kind, was by no 

 means new to Darwin, for the elder De Candolle, 

 Lyell and others had enlarged upon it ; yet the facts 

 with regard to the human race, so strikingly pre- 

 sented by Malthus, brought the whole question with 

 such vividness before him that the idea of ' Natural 

 Selection* flashed upon Darwin's mind. This hypo- 

 thesis cannot be better or more succinctly stated 

 than in Huxley's words. 



'All species have been produced by the development of 

 varieties from common stocks : by the conversion of these, first 

 into permanent races and then into new species, by the process 

 of natural selection, which process is essentially identical with 

 that artificial selection by which man has originated the races of 

 domestic animals the struggle for existence taking the place of 

 man, and exerting, in the case of natural selection, that selective 

 action which he performs in artificial selection 108 .' 



With characteristic caution, Darwin determined 

 not to write down ' even the briefest sketch ' of this 

 hypothesis, that had so suddenly presented itself to 



