330 The Theory of Natural Selection 



thinking on both problems has been obstructed. It is neces- 

 sary for our purpose to keep distinct the general theory of 

 organic evolution and Darwin's explanation, even if we learn 

 about both out of the same book, The Origin of Species, 



Malfhus as Catalyzer 



Darwin tells us in his Autobiography that after his re- 

 turn from a voyage around the earth as naturalist for The 

 Beagle he started a set of note books on variations in plants 

 and animals which had impressed him in the course of his 

 journey. For twenty years he gathered material to prove 

 that there had been descent with modification. He hesi- 

 tated about publishing his results because he could not make 

 up his mind just hoiv transmutation of forms had come 

 about. His health was such that he was compelled to live 

 quietly in the country, being able to work only a few hours 

 a day. Apparently, however, he was able to accomplish a 

 great deal through reading, correspondence and observation 

 of the living things in his garden and in the barnyard. 



With the problems constantly on his mind, groping for 

 some principle that would unify his vast accumulation of 

 direct observations and of the recorded observations of 

 others, he chanced to read Malthus's Essay on Population. 

 The immediate effect of this, Darwin writes, was to clarify 

 the whole problem. He proceeded immediately to shape his 

 now famous theory of natural selection. The stimulating 

 thought which he got from Malthus furnished Darwin the 

 element of his theory which he called ** the struggle for ex- 

 istence." Malthus pointed out that the tendency of each 

 species of living things (particularly, for his purpose, of 

 human beings) is to multiply in a geometrical ratio. The 

 means of subsistence, on the contrary, can multiply only in 

 an arithmetical ratio. We need not concern ourselves for 

 the present with the fact that Malthus borrowed his ideas 

 from Benjamin Franklin. Nor with the further fact that 

 he got his facts wrong. The immediate result for Darwin 



