"I grieve to differ from you, and it actually terrifies me, and
makes me constantly distrust myself. I fear we shall never quite
understand each other."

--DARWIN TO WALLACE.

During the period covered by the reception, exposition, and gradual
acceptance of the theory of Natural Selection, both Wallace and Darwin
were much occupied with closely allied scientific work.

The publication in 1859 of the "Origin of Species"[1] marked a distinct
period in the course of Darwin's scientific labours; his previous
publications had, in a measure, prepared the way for this, and those
which immediately followed were branches growing out from the main line
of thought and argument contained in the "Origin," an overflow of the
"mass of facts" patiently gathered during the preceding years. With
Wallace, the end of the first period of his literary work was completed
by the publication of his two large volumes on "The Geographical
Distribution of Animals," towards which all his previous thought and
writings had tended, and from which, again, came other valuable works
leading up to the publication of "Darwinism" (1889).

It will be remembered that Darwin and Wallace, on their respective
returns to England, after many years spent in journeyings by land and
sea and in laborious research, found the first few months fully occupied
in going over their large and varied collections, sorting and arranging
with scrupulous care the rare specimens they had taken, and in
discovering the right men to name and classify them into correct groups.

At this point it will be useful to arrange Darwin's writings under three
heads, namely: (1) His zoological and geological books, including "The
Voyage of the _Beagle_" (published in 1839), "Coral Reefs" (1842), and
"Geological Observations on South America" (1846). In this year he also
began his work on Barnacles, which was published in 1854; and in
addition to the steady work on the "Origin of Species" from 1837
onwards, his observations on "Earthworms," not published until 1881,
formed a distinct phase of his study during the whole of these years
(1839-59). (2) As a natural sequence we have "Variations of Animals and
Plants under Domestication" (1868), "The Descent of Man" (1871), and
"The Expression of the Emotions" (1872). (3) What may be termed his
botanical works, largely influenced by his evolutionary ideas, which
include "The Fertilisation of Orchids" (1862), "Movements and Habits