72 LIFE OF 



that man's power of selecting and breeding from certain 

 individuals was the most powerful of all means in the 

 production of new races. Having attended to the habits 

 of animals, and their relations to the surrounding con- 

 ditions, I was able to realise the severe struggle for 

 existence to which all organisms are subjected ; and my 

 geological observations had allowed me to appreciate, to 

 a certain extent, the duration of past geological periods. 

 With my mind thus prepared, I fortunately happened to 

 read Malthus's ' Essay on Population ; ' and the idea of 

 natural selection through the struggle for existence at 

 once occurred to me. Of all the subordinate points in 

 the theory, the last which I understood was the cause of 

 the tendency in the descendants from a common pro- 

 genitor to diverge in character." x 



Malthus taught the inevitable tendency of all animal 

 life to increase beyond the means of subsistence, and 

 expounded the checks which begin to act when popula- 

 tion increases too rapidly. But his book had lain un- 

 fruitful to naturalists since 1798, until Darwin read it, 



volume of the " Horticultural Transactions," and in his work on 

 the Amaryllidaceoe, 1837, declared that horticultural experiments 

 have established, beyond the possibility of refutation, that botanical 

 species are only "a higher and more permanent class of varieties." 

 He extended the same view to animals, and believed that single 

 species of each genus were originally created in a highly plastic 

 condition, and that these have produced, chiefly by intercrossing, 

 but also by variation, all our existing species. 



'The first portion of this important letter is quoted from the 

 English translation of Haeckel's " History of Creation," 1876 ; the 

 second portion from O. Schmidt's "Doctrine of Descent and 

 Darwinism," having been re-written by Darwin from the German 

 text. 



