338 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1958 
The main steps in Darwin’s proof of the fact of evolution were 
established by 1842, when he committed them to paper in the form 
of a Sketch which he expanded into an Essay in 1844, though neither 
was published by him. Soon after this, another naturalist, Alfred 
Russel Wallace, was led to explore similar lines of research. From 
some simple observations on the distribution of organisms, both geo- 
graphically over the world and geologically in the fossil record, 
Wallace drew some equally simple conclusions that are of great im- 
portance in the history of thought that led to the realization of 
evolution. They show that, independently of Darwin and in com- 
plete ignorance of his work, Wallace had hit upon the same solution 
of the problem of the mutability of species. 
Wallace’s observations were based on the facts, first, that large 
systematic groups such as classes and orders are usually distributed 
over the whole of the earth, whereas groups of low systematic value 
such as families, genera, and species frequently have a very small 
localized distribution. Second, “when a group is confined to one dis- 
trict, and is rich in species, it is almost invariably the case that the 
most closely allied species are found in the same locality or in closely 
adjoining localities, and that therefore the natural sequence of the 
species by aflinity is also geographical.” Third, in the fossil record 
large groups extend through several geological formations, and “no 
group or species has come into existence twice.” 
The conclusion which Wallace drew from these observations was 
that “Every species has come into existence coincident both in space 
and time with a pre-existing closely allied species.” Thought out 
about 1845, written at Sarawak in 1855, and published in the same 
year, Wallace’s theory already allowed him to say that “the natural 
series of affinities will also represent the order in which the several 
species came into existence, each one having had for its immediate 
antitype a closely allied species existing at the time of its origin. It 
is evidently possible that two or three distinct species may have had 
a common antitype, and that each of these may again have become 
the antitype from which other closely allied species were created.” 
With the help of this principle, in which it is only necessary to 
substitute “ancestor” for “antitype” for the formulation of evolution 
to be complete, Wallace showed that it was possible to give a simple 
explanation of natural classification, of the geographical distribu- 
tion of plants and animals, including those of the Gal4pagos Islands, 
of the succession of forms in the fossil record, and of rudimentary 
organs which would be inexplicable “if each species had been created 
independently, and without any necessary relations with preexisting 
Species.” 
