442 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
formed flat islands. Darwin recognized that the process could not 
be quite so simple, because the polyps can not live at great depths. 
He therefore assumed that a secular subsidence of the ocean floor 
must have played a part, and this hypothesis not only explains in 
the most beautiful way the details of the structure of an atoll, but 
it has been brilliantly corroborated by later investigations, especially 
by borings on one of the islands, and the theory is now a permanent 
possession of science. After the completion of this volume he worked 
for eight years at the rich material he had brought from the coast of 
Chile, of that remarkable group of sedentary crustaceans, the Cirri- 
pedes, usually known as barnacles and acorn shells. Two thick 
volumes on this subject appeared in 1851, and later two other quarto 
volumes on fossil species of the same group. Even here, in this appar- 
ently dry and purely systematic province, the true spirit of the in- 
vestigator revealed itself, for he did not neglect what was unintelligi- 
ble to him, and therefore inconvenient for his theory, but devoted the 
-most persistent attention to obscure points until he had found a 
solution of the difficulty. Thus he discovered that within the group 
there are species which, like all Cirripedes, are hermaphrodite, but 
which possess in addition small degenerate-looking males of dif- 
ferent structure attached as parasites to the hermaphrodite animals. 
It is, however, only in our own day that it has become possible to 
understand the deeper significance of this important discovery. 
In addition to these special pieces of work Darwin collected with 
untiring energy facts which had any bearing on the theory of trans- 
mutation, having begun in 1837, just after his return to England, a 
large collecting notebook, in which he entered all the facts referring 
to the variability of animals and plants, in particular of those which 
are under the care of man. By means of printed lists of questions, of 
conversations with expert breeders of animals and plants, and of 
wide reading in books and journals, he sought to lay the foundation 
of fact which he required in order to attain to clearness in regard to 
the supposed transformation of organisms. 
He was very soon led to the conviction that the essential factor in 
the artificial modification of an animal or plant form was selection 
for breeding. But how could such selection take place in free nature é 
For a long time he was unable to find the answer to this question, 
until chance. made him acquainted with the work of the economist 
Malthus on “ Population,” and the ideas developed in this book sug- 
gested to him the solution of the problem. Malthus showed that the 
human population multiplied much more rapidly than the means of 
subsistence could increase, and that therefore catastrophes must occur 
from time to time to diminish the excessive number of human beings. 
Darwin said to himself that in the rest of nature, among other forms 
of life also, an enormous number of individuals must perish, since 
