This letter was finished after his arrival at Ternate, and a few weeks later he was prostrated by a sharp attack of intermittent fever which obliged him to take a prolonged rest each day, owing to the exhausting hot and cold fits which rapidly succeeded one another.

The little bungalow at Ternate had now come to be regarded as “home” for it was here that he stored all his treasured collections, besides making it the goal of all his wanderings in the Archipelago.  One can understand, therefore, that, in spite of the fever, there was a sense of satisfaction in the feeling that he was surrounded with the trophies of his arduous labours as a naturalist, and this passion for species and their descriptions being an ever-present speculation in his mind, his very surroundings would unconsciously conduce towards the line of thought which brought to memory the argument of “positive checks” set forth by Malthus in his “Principles of Population” (read twelve years earlier) as applied to savage and civilised races.  “It then,” he says, “occurred to me that these causes or their equivalents are continually acting in the case of animals also; and as animals usually breed much more rapidly than does mankind, the destruction every year from these causes must be enormous in order to keep down the numbers of each species, since they evidently do not increase regularly from year to year, as otherwise the world would have been densely crowded with those that breed most quickly....  Then it suddenly flashed upon me that this self-acting process would necessarily improve the race, because in every generation the inferior would inevitably be killed off and the superior would remain—­that is, the fittest would survive.  Then at once I seemed to see the whole effect of this, that when changes of land and sea, or of climate, or of food-supply, or of enemies occurred—­and we know that such changes have always been taking place—­and considering the amount of individual variation that my experience as a collector had shown me to exist, then it followed that all the changes necessary for the adaptation of the species to the changing conditions would be brought about; and as great changes in the environment are always slow, there would be ample time for the change to be effected by the survival of the best fitted in every generation.  In this way every part of an animal’s organism could be modified as required, and in the very process of this modification the unmodified would die out, and thus the definite characters and the clear isolation of each new species would be explained.  The more I thought over it the more I became convinced that I had at length found the long-sought-for law of nature that solved the problem of the origin of species.  For the next hour I thought over the deficiencies in the theories of Lamarck and of the author of the ‘Vestiges,’ and I saw that my new theory supplemented these views and obviated every important difficulty.  I waited anxiously for the termination of my fit (of fever) so that I might at once make notes for a paper on the subject.  The same evening I did this pretty fully, and on the two succeeding evenings wrote it out carefully in order to send it to Darwin by the next post, which would leave in a day or two."[23]