458 TROPICAL NATURE Ix 
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 
