134 THE HISTORY OF CREATION. 
trate his attention upon the great problem to which his 
mind had been turned during his voyage in the Beagle. In 
order to show what kind of observations during the voyage 
principally gave rise to the fundamental idea of the Theory 
of Selection, and in what manner he afterwards worked 
it out, I shall insert here a passage from a letter which he 
addressed to me on the 8th of October, 1864. 
Letter from Charles Darwin to Haeckel, 8th October, 1864. 
“Tn South America three classes of facts were brought 
strongly before my mind. Firstly, the manner in which 
closely allied species replace species in going southward. 
Secondly, the close affinity of the species inhabiting the 
islands near South America to those proper to the con- 
tinent. This struck me profoundly, especially the differ- 
ence of the species in the adjoining islets in the Galopagos. 
Archipelago. Thirdly, the relation of the living Edentata. 
and Rodentia to the extinct species. I shall never forget 
my astonishment when I dug out a gigantic piece of armour 
like that of the living armadillo. 
“ Reflecting on these facts, and collecting analogous ones, it 
seemed to me probable that allied species were descended 
from a common parent. But for some years I could not 
conceive how each form became so excellently adapted to 
its habits of life. I then began systematically to study 
domestic productions, and after a time saw clearly that 
man’s selective power was the most important agent. I was. 
prepared, from having studied the habits of animals, to ap- 
preciate the struggle for existence, and my work in geology 
gave me some idea of the lapse of past time. Therefore, 
when I happened to read “ Malthus on Population,” the idea. 
