430 Experimental Zoology 
the central nervous system also. We find these organs most per- 
fectly developed in the higher animals, hence it may appear that 
we can explain on this basis the occurrence of secondary sexual 
organs in these groups, and the absence of such differences be- 
tween the sexes in the lower groups with less developed sense 
organs. 
Before we examine critically these and other hypotheses it 
may be well first to pass in review some examples of secondary 
characters, and then examine the experimental evidence that 
shows how far the development of these characters is dependent 
on the presence of the essential organs of reproduction." 
In the mammals there are many excellent examples of differ- 
ences between the male and the female. In the mandrill the 
naked skin of the face of the male is bright blue, with the tip 
and sides of the nose a brilliant red. The face is also marked 
with white stripes. On the forehead there is a crest of hair and 
on the chin a yellow beard. In the monkey Cercopithecus diana 
the head of the male is intensely black, while that of the female 
is gray. The enormous development of antlers in the stag is a 
striking example of a secondary sexual difference. 
In birds there are many species in which the males are entirely 
differently colored from the females. The scarlet tanager is 
a familiar example. The male during the pairing season is 
a brilliant scarlet, with jet-black wings, while the female is a 
modest yellow-green. The Baltimore oriole and the orchard 
orioles show almost as great differences in the coloration of the 
two sexes. Many of the humming birds show extraordinary 
differences between the male and female, and the birds of para- 
dise and the peacock excel in this respect. 
* Darwin brought together in his book on “The Descent of Man” a great num- 
ber of cases in which secondary sexual differences occur. Wallace also, on several 
occasions, has given examples of the same sort. Scattered through the system- 
atic literature of the groups of mammals, birds, and insects a vast number of 
cases are described. Recently Cunningham has also brought together a large 
number of cases of secondary differences and has suggested another way for 
accounting for them, as will be stated later. Lameere has also given a number 
of examples, especially among insects. 
