88 CHARLES DARWIN. 



before but not then published, and as it has thus an 

 historical interest, a few words of personal statement may 

 be permissible. After writing the preceding paper [" On 

 the Law which has Regulated the Introduction of New 

 Species"] the question of how changes of species could have 

 been brought about was rarely out of my mind, but no 

 satisfactory conclusion was reached till February 1858. At 

 that time I was suffering from a rather severe attack of 

 intermittent fever at Ternate in the Moluccas, and one day, 

 while lying on my bed during the cold fit, wrapped in 

 blankets, though the thermometer was at 88° Fahr., the 

 problem again presented itself to me, and something led me to 

 think of the 'positive checks' described by Malthus in his 

 'Essay on Population,' a work I had read several years 

 before, and which had made a deep and permanent impression 

 on my mind. These checks — war, disease, famine and the like — 

 must, it occurred to me, act on animals as well as man. Then 

 I thought of the enormously rapid multiplication of animals, 

 causing these checks to be much more effective in them than 

 in the case of man ; and while pondering vaguely on this fact 

 there suddenly flashed upon me the idea of the survival of the 

 fittest— that the individuals removed by these checks must be 

 on the whole inferior to those that survived. In the two hours 

 that elapsed before my ague fit was over I had thought out 

 almost the whole of the theory, and the same evening I 

 sketched the draft of my paper, and in the two succeeding 

 evenings wrote it out in full, and sent it by the next post to 

 Mr. Darwin. Up to this time tlie only letters I had received 

 from him were those printed in the second volume of his 

 Life and Letters (vol. ii., pp. 95 and 108), in which he speaks 

 of its being the twentieth year since he ' opened his first 

 note-book on the question how and what way do species and 

 varieties differ from each other,' and after referring to oceanic 

 islands, the means of distribution of land-shells, (fee, added : 

 ' My work, on which I have now been at work more or less for 

 twenty years, will not fix or settle anything ; but I hope it 

 will aid by giving a large collection of facts, with one definite 

 end.' The words I have italicised, and the whole tone of his 

 letters, led me to conclude that he had arrived at no definite 

 view as to the origin of species, and I fully anticipated that. 



