444 Experimentat Zoology 
are more difficult to change than color; but Wallace brings 
forward no evidence to establish this view. 
In other vertebrates this hypothesis will not explain the plainer 
coloration of the females, as Wallace points out, since they do not 
incubate their eggs. Secondary sexual differences are known in 
fishes, lizards, and mammals that appear to be similar to those 
in birds. Wallace has recourse, therefore, to the theory of 
natural selection amongst the males which has led to the sur- 
vival of the stronger males. In polygamous races this leads to the 
successful males becoming the fathers of the next generation. 
The stronger animals are those whose colors and other such char- 
acters will be more highly developed. In monogamous species, 
and these are by far the most common, the result of competition 
amongst the males will not produce any result, since the van- 
quished males will also find mates. Wallace tries to meet this 
difficulty by pointing out that the males of many insects emerge 
before the females, and that the males of migratory birds are the 
first to arrive at the nesting grounds. The most vigorous males 
will be the first to pair and leave offspring. This precocity is 
assumed to be an advantage, but it is obvious that it might be 
also a disadvantage, and we have no means of weighing the rela- 
tive merits of the individuals that pair first with those that pair 
at the height of the breeding season. 
Wallace thinks that the greater vigor of the males will lead 
in itself to the higher coloration of that sex, and furthermore he 
tries to show that the ornaments are more likely to occur over 
those parts of the animal where the muscular and nervous de- 
velopment is greatest; but the location of the tail of the peacock, 
the crest of the herons, and many other characters that might be 
cited would seem opposed to such an assumption. 
Darwin has considered Wallace’s view in his ‘Descent of 
Man” and has pointed out that while the theory might appear 
to apply to sexual differences in color and ornamentation, it is 
not obvious how other secondary sexual characters, such as the 
vocal organs of birds and the scent glands of mammals, could be 
thus explained. Wallace has replied to this criticism by assuming 
