XXII] CELEBES, THE MOLUCCAS, ETC. 361 



allies by slight yet perfectly definite and constant characters. 

 One would expect that if it was a law of nature that species 

 were continually changing so as to become in time new and 

 distinct species, the world would be full of an inextricable 

 mixture of various slightly different forms, so that the 

 well-defined and constant species we see would not exist. 

 Again, not only are species, as a rule, separated from each 

 other by distinct external characters, but they almost always 

 differ also to some degree in their food, in the places they 

 frequent, in their habits and instincts, and all these characters 

 are quite as definite and constant as are the external characters. 

 The problem then was, not only how and why do species 

 change, but how and why do they change into new and well- 

 defined species, distinguished from each other in so many 

 ways ; why and how do they become so exactly adapted to 

 distinct modes of life ; and why do all the intermediate grades 

 die out (as geology shows they have died out) and leave only 

 clearly defined and well-marked species, genera, and higher 

 groups of animals. 



Now, the new idea or principle which Darwin had arrived 

 at twenty years before, and which occurred to me at this time, 

 answers all these questions and solves all these difficulties, 

 and it is because it does so, and also because it is in itself 

 self-evident and absolutely certain, that it has been accepted 

 by the whole scientific world as affording a true solution of 

 the great problem of the origin of species. 



At the time in question I was suffering from a sharp 

 attack of intermittent fever, and every day during the cold 

 and succeeding hot fits had to lie down for several hours, 

 during which time I had nothing to do but to think over any 

 subjects then particularly interesting me. One day something 

 brought to my recollection Malthus's " Principles of Popula- 

 tion," which I had read about twelve years before. I thought 

 of his clear exposition of " the positive checks to increase " — 

 disease, accidents, war, and famine — which keep down the 

 population of savage races to so much lower an average than 

 that of more civilized peoples. It then occurred to me that 

 these causes or their equivalents are continually acting in the 



