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 
