SEXUAL COLOUR 83 
explains the dull colour of females but not 
the brilliant colour of males. 
Emery’s theory of the origin of secondary 
sexual characters as mono-sexual sports may 
explain their origin, although there appears 
to be little evidence of this, but makes no 
attempt to define the cause of these differ- 
ences ; or assumes that the sport is retained 
because and when it fits the environment 
better than the old type. 
Those who believe in the transmission of ac- 
quired characters look upon secondary sexual 
characters as adaptations and due to the 
stimulation of parts through use. Controlled 
natural selection accounts for both the origin 
and purpose of secondary sexual characters 
in the following way. Males are more con- 
spicuous in nature than females: males are 
less valuable than females. Males and females 
are associated together during life, and especi- 
ally during the breeding season when the 
difference in colour is greatest, and when their 
difference in value is highest : therefore accord- 
ing to the theory, the conspicuous colour of 
the male serves to control natural selection 
in such a way that the less valuable male 
will be killed in preference to the more 
valuable female. 
