CELEBES, THE MOLUCCAS, ETC. 361 



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 exter- 

 nal 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 

 case of animals also ; and as animals usually breed much more 

 rapidly than does mankind, the destruction every year from 



