Ix THE DEBT OF SCIENCE TO DARWIN 459 
the cirripedia or barnacles, as well as of a most ingenious 
explanation of the origin and structure of coral-reefs—a 
series of volumes which were the direct outcome of his 
voyage, and which gave him an established reputation. Even 
when the great work at last appeared, few could appreciate 
the enormous basis of fact and experiment on which it rested, 
until, during the succeeding twenty years, there appeared 
that remarkable succession of works which exhibited a sample 
(and only a sample) of the exhaustless store of materials and 
the profound maturity of thought on which his early volume 
was founded. From these various works, aided by some per- 
sonal intercourse and a correspondence extending over twenty 
years, the present writer will endeavour to indicate the 
nature and extent of Darwin’s researches. 
Studies of Domestic Animals 
Although, as we have said, Darwin had early arrived at 
the conclusion that allied species had descended from common 
ancestors by gradual modification, it long remained to him an 
inexplicable problem how the necessary degree of modifica- 
tion could have been effected, and he adds: “It would thus 
have remained for ever, had I not studied domestic produc- 
tions, and thus acquired a just idea of the power of selection.” 
These researches, very briefly sketched in the first and parts 
of the fifth and ninth chapters of the Origin of Species, were 
published at length (after a delay of nine years, owing to ill 
health) in two large volumes, with the title Animals and Plants 
Under Domestication ; and no one who has not read these 
can form an adequate idea of the wide range and thorough 
character of the investigation on which every statement or 
suggestion in the former work was founded. 
The copious references to authorities show us that he 
must have searched through almost the entire literature of 
agriculture and horticulture, of horse and cattle breeding, of 
sporting, of dog, cat, pigeon, and fowl fancying, including 
endless series of reviews, magazines, journals of societies, and 
newspapers, besides every scientific treatise bearing in any 
way on the subject, whether published in this country, on the 
Continent, or in America. The facts thus laboriously gathered 
were supplemented by personal inquiries among zoologists and 
