98 THE EVOLUTION OF MAN. 



phical distribution of species, and on the other, the relation 

 between the living and extinct species of the same continent, 

 suggested to him the idea that nearly allied species might 

 have descended from a common ancestral form. On his 

 return from his five years' voyage, he devoted himself for 

 years most zealously to the systematic study of domestic 

 animals and garden-plants, and he recognized the evident 

 analogies between the formation and transmutations of these, 

 and those of wild species in a state of nature. He had, 

 however, no conception of natural selection through the 

 struggle for existence, which is the most important feature 

 in the construction of his theory, until after he had read 

 the celebrated book of Malthus, the political economist, on 

 the " Principles of Population." This gave him a clear 

 conception of the analogy between the changing relations 

 of population and over-population in civilized countries and 

 the social relations of animals and plants in a wild state. 

 He continued for many years to collect materials in order to 

 accumulate a great mass of evidence for the support of this 

 theory. At the same time, as a practical breeder, he insti- 

 tuted many important experiments in breeding, and gave 

 special attention to the instructive breeding of domestic 

 pigeons. Ample leisure was afforded him by the quiet 

 retirement in which, after his return from his journey 

 round the world, he has lived on his property of Down, near 

 Beckenham. 



It was not until the year 1858, that Darwin was induced, 

 by the work of another naturalist, Alfred Russell Wallace, 

 who had conceived the same Theory of Selection, to publish 

 the outlines of his theory. In 1859 appeared his principal 

 work, " On the Origin of Species," in which the theory is 



