DARWIN AND WALLACE 35 



of men; for he was to promulgate a theory that would 

 arouse the bitterest opposition and the keenest scorn. 

 All the while Darwin was working on these books 

 his mind was quietly busying itself with what he 

 called the species question. The more he studied the 

 material collected on his long tour, the more confident 

 he became that the animals of the present are the 

 altered descendants of the animals of the past. He 

 tried patiently to work out every conceivable hypoth- 

 esis to see whether he could account for the alteration. 

 He felt quite sure animals changed, but how they 

 changed, and why, he could not for a long time con- 

 ceive. He knew that gardeners were constantly pro- 

 ducing new varieties of plants, and that animals of va- 

 rious breeds were clearly the descendants of other and 

 familiar varieties. Accordingly he began to study the 

 methods of animal and plant breeders, to visit their 

 farms, to open correspondence with them and read all 

 their trade journals, to undertake experiments in the 

 breeding of plants. The longer he worked the more 

 confident he became of the reality of the change; but 

 for a long time no glimmer of the cause by which it 

 could be brought about came to his mind. In 1838 

 he came across a book by Malthus called "An Essay 

 on Population," in which the author shows that, 

 whereas man increases by a geometric ratio, he can- 

 not hope to increase his food supply in more than an 



