The Making of Species 



larynx in some birds and mammals. We thus see 

 that suggestions offered by Wallace, although 

 they contain a modicum of truth, fail to explain 

 the phenomena of sexual dimorphism. 



The fairest possible criticism of these views is 

 that of Darwin : 



"It will have been seen that I cannot follow 

 Mr Wallace in the belief that dull colours, when 

 confined to the females, have been in most cases 

 specially gained for the sake of protection. 

 There can, however, be no doubt, as formerly 

 remarked, that both sexes of many birds have 

 had their colours modified, so as to escape the 

 notice of their enemies ; or in some instances, so 

 as to approach their prey unobserved, just as 

 owls have had their plumage rendered soft, that 

 their flight may not be overheard " ( The Descent 

 of Man, p. 745). 



THE THEORY OF THOMSON AND GEDDES 



Thomson and Geddes have attempted to 

 explain sexual dimorphism on the hypothesis 

 that males are essentially dissipators of energy, 

 while females tend to conserve energy. They 

 point out that the spermatozoon is a small 

 intensely active body, which dissipates its energy 

 in motion, while the ovum is a large inert body 

 the result of the female tendency to conserve 

 energy and to build up material. The various 

 ornaments and excrescences which appear in 



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