458 TROPICAL NATURE 



He also saw, at this early period, the important fact that 

 there is some great and constant check to the increase of wild 

 animals, though most of them breed very rapidly, and, of 

 course, would increase in a geometrical ratio were some such 

 check not in constant action. He traces the comparative 

 rarity of a species to less favourable conditions of existence, 

 and extinction to the normal action of still more unfavourable 

 conditions, and compares the destruction of a species by man 

 and its extinction by its natural enemies as being phenomena 

 of the same essential nature. The various classes of facts 

 here referred to seemed to him " to throw some light on the 

 origin of species that mystery of mysteries, as it has been 

 called by one of our greatest philosophers ; " and he tells us 

 that, soon after his return home in 1837, it occurred to him 

 " that something might perhaps be made out on this question 

 by patiently accumulating and reflecting on all sorts of facts 

 which could possibly have any bearing upon it." We know 

 from his own statement that he had already perceived that no 

 explanation but some form of the derivation or development 

 hypothesis, as it was then termed, would adequately explain 

 the remarkable facts of distribution and geological succession 

 which he had observed during his voyage ; yet he tells us 

 that he worked on for five years before he allowed himself to 

 speculate on the subject; and then, having formulated his 

 provisional hypothesis in a definite shape during the next 

 two years, he devoted another fifteen years to continuous 

 observation, experiment, and literary research, before he gave 

 to the astounded scientific world an abstract of his theory in 

 all its wide-embracing scope and vast array of evidence, in his 

 epoch-making volume, The Origin of Species. 



If we add to the periods enumerated above the five years' 

 observation and study during the voyage, we find that this 

 work was the outcome of twenty -seven years of continuous 

 thought and labour, by one of the most patient, most truth- 

 loving, and most acute intellects of our age. During all this 

 long period only a very few of his most intimate friends were 

 aware that he had departed from the then beaten track of 

 biological study, while the great body of naturalists only 

 knew him as a good geologist, as the writer of an interesting 

 book of travels, and the author of an admirable monograph of 



