em; and in certain
fish, and probably in other animals, an analogous heightening of effect
accompanies nervous excitement other than sexual, such as that due to
fighting or feeding. Although there is epigamic display in species with
sexes alike, it is usually most marked in those with secondary sexual
characters specially developed in the male. These are an exception to
the rule in heredity, in that their appearance is normally restricted to
a single sex, although in many of the higher animals they have been
proved to be latent in the other, and may appear after the essential
organs of sex have been removed or become functionless. This is also the
case in the Aculeate Hymenoptera when the reproductive organs have been
destroyed by the parasite _Stylops_. J. T. Cunningham has argued
(_Sexual Dimorphism in the Animal Kingdom_, London, 1900) that secondary
sexual characters have been produced by direct stimulation due to
contests, &c., in the breeding period, and have gradually become
hereditary, a hypothesis involving the assumption that acquired
characters are transmitted. Wallace suggests that they are in part to be
explained as "recognition characters," in part as an indication of
surplus vital activity in the male.

AUTHORITIES.--The following works may also be consulted:--T. Eimer,
_Orthogenesis der Schmetterlinge_ (Leipzig, 1898); E. B. Poulton, _The
Colours of Animals_ (London, 1890); F. E. Beddard, _Animal Coloration_
(London, 1892); E. Haase, _Researches on Mimicry_ (translation,
London, 1896); A. R. Wallace, _Natural Selection and Tropical Nature_
(London, 1895); _Darwinism_ (London, 1897); A. H. Thayer and G. H.
Thayer, _Concealing-Coloration in the Animal Kingdom_ (New York,
1910). (E. B. P.)

2. CHEMISTRY

The coloration of the _surface_ of animals is caused either by
_pigments_, or by a certain _structure_ of the surface by means of which
the light falling on it, or reflected through its superficial
transparent layers, undergoes diffraction or other optical change. Or it
may be the result of a combination of these two causes. It plays an
important part in the relationship of the animal to its environment, in
concealment, in mimicry, and so on; the presence of a pigment in the
integument may also serve a more direct physiological purpose, such as a
respiratory function. The coloration of birds' feathers, of the skin of
many fishes, of many insects, is partially at least due to structu