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 of
Climbing Plants" (1875), "Insectivorous Plants" (1876), "The Different
Forms of Flowers and Plants of the same Species" (1877), and "The Power
of Movement in Plants" (1880).

A different order, equally characteristic, is discovered in Wallace's
writings, and it is to be noted that while Darwin devoted himself
entirely to scientific subjects, Wallace diverged at intervals from
natural science to what may be termed the scientific consideration of
social conditions, in addition to his researches into spiritualistic
phenomena.

The many enticing interests arising out of the classifying of his birds
and insects led Wallace to the conclusion that it would be best to
postpone the writing of his book on the Malay Archipelago until he could
embody in it the more generally important results derived from the
detailed study of certain portions of his collections. Thus it was not