66 
Introduction: Variation. 
fulfil the usual maternal duties properly; the males of the Swallow, Goldfinch 
and Hoopoe which, though very like their mates, are said to take no share in 
the incubation of the eggs; while the male Eeed Bunting and Blackcap, which 
are more highly developed than their mates, nevertheless help a little in hatching 
the eggs. There appears, therefore, to be no hard and fast law of correlation 
in the evolution of higher organic development and of mental functions of the 
“masculine” type; in other words, the structural differences and the psychological 
differences of the sexes seem to have been developed independently. 
Theories in ewplanation of the development of secondary sexual characters. ■ — 
Several have been advanced: 
1. Darwin (Descent of Man, 1871, pp. 38 — 238, Birds) accounts for the 
superiority of the male by reason of the choice by the female of the male which 
pleases her best (sexual selection), and 
2. partly by the survival of the fittest in combat. 
3. Wallace (Darwinism, 1889, 289, et seq.) believes that the secondary 
sexual differences have risen to a higher development in one sex owing to a 
prepotency of vitality or growth-power, and some evidence is adduced tending to 
make it plausible that the accessory plumes of the males are developed over 
centres of high muscular or nervous activity. 
4. Wallace (t. c. p. 277, Darwin, t. c. p. 166) adds the complementary theory 
that the need for protective coloration in the brooding female has prevented 
her by natural selection from acquiring many of the bright colours and showy 
ornaments of the male. 
5. Stolzmann (P. Z. S. 1885, 421, et seq.) bases a theory on the assump- 
tion that among birds the males are more numerous than the females. Any 
development of colour or markings which is disadvantageous to the males, by 
rendering them more conspicuous and more easily destructible to foes (whether 
predaceous animals or males of their own species), will be advantageous for the 
species, because the superfluous males are parasites devouring food which would 
be useful to the breeding birds, persecutors of the brooding females, destroyers 
of the eggs, etc. It is argued that natural selection will favour the preservation 
of those females which produce male offspring handicapped with such peculiari- 
ties of structure, plumage, temperament, etc., as are likely to bring about the 
destruction of these males. 
6. Beddard (Animal Coloration, 1892, p. 282 et antea) finds that “the 
secondary sexual characters of animals are dependent upon the germ glands 
themselves; and that the sexual diversity of animals is also associated with 
differences of disposition and habit”. 
Touching Darwin’s theories it is obvious that natural selection in the “law 
of battle” affords a simple explanation of the development of certain offensive 
and defensive organs, greater size, strength, activity and courage. 
Mr. Wallace’s theory of the development of protective coloration in the 
