Q^ Introduction: Variation. 



fulfil the usual maternal duties pro])erly; 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 Reed 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" tyi)e; in other words, the structural differences and the psychological 

 differences of the sexes seem to have been developed independently. 



Theories in explmtation 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 

 ])leases 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 sujjerfluous 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 handicajjped 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 ai"e 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 



