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 :— 
“Tt 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” (7he Descent 
of Man, p. 745). 
Tue THrEoRY 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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