Contributions to the theory of natural selection. A series of essays ... By Alfred Russel Wallace ...

RESEMBLANCES AMONG ANIMALJS. 111
tible with either of the other hypotheses; namely, the
relation of protective colouring and mimicry to the
sexual differences of animals. It will be clear to every
one that if two animals, which as regards " external
conditions" and " hereditary descent," are exactly
alike, yet differ remarkably in colouration, one resembling a protected species and the other not, the resemblance that exists in one only can hardly be imputed
to the influence of external conditions or as the effect
of heredity. And if, further, it can be proved that
tlie one requires protection more than the other, and
that in several cases it is that one which mimics the
protected species, while the one that least requires
protection never does so, it will afford very strong
corroborative evidence that there is a real connexion
between the necessity for protection and the phenomenon of mimicry. Now the sexes of insects offer
us a test of the nature here indicated, and appear to
furnish one of the most conclusive arguments in favour
of the theory that the phenomena termed " mimicry"
are produced by natural selection.
The comparative importance of the sexes varies
much in different classes of animals. In the higher
vertebrates, where the number of young produced at a
birth is small and the same individuals breed many
years in succession, the preservation of both sexes is
almost equally important. In all the numerous cases
in which the male protects the female and her offspring,
or helps to supply them with food, his importance in
the economy of nature is proportionately increased,