His one object, as set out in the Preface, was to treat the problem of
the origin of species from the standpoint reached after nearly thirty
years of discussion, with an abundance of new facts and the advocacy of
many new and old theories. As it had frequently been considered a
weakness on Darwin's part that he based his evidence primarily on
experiments with domesticated animals and cultivated plants, Wallace
desired to secure a firm foundation for the theory in the variation of
organisms in a state of nature. It was in order to make these facts
intelligible that he introduced a number of diagrams, just as Darwin was
accustomed to appeal to the facts of variation among dogs and pigeons.

Another change which he considered important was that of taking the
struggle for existence first, because this is the fundamental phenomenon
on which Natural Selection depends. This, too, had a further advantage
in that, after discussing variations and the effects of artificial
selection, it was possible at once to explain how Natural Selection
acts.

The subjects treated with novelty and interest in their important
bearings on the theory of Natural Selection were: (1) A proof that all
_specific_ characters are (or once have been) either useful in
themselves or correlated with useful characters (Chap. VI.); (2) a proof
that Natural Selection can, in certain cases, increase the sterility of
crosses (Chap. VII.); (3) a fuller discussion of the colour relations of
animals, with additional facts and arguments on the origin of sexual
differences of colour (Chaps. VIII.-X.); (4) an attempted solution of
the difficulty presented by the occurrence of both very simple and
complex modes of securing the cross-fertilisation of plants (Chap. XI.);
(5) some fresh facts and arguments on the wind-carriage of seeds, and
its bearing on the wide dispersal of many arctic and alpine plants