190 NATURAL SCIENCE [September 
metamorphosis to which I shall refer further on. At present I will 
pass on to the consideration of the second class of structural 
differences, those which distinguish constant forms within one 
species. 
The commonest and most widely extended case of this is the 
existence of what are called secondary sexual characters, in other 
words the existence of structural differences between males and 
females in addition to the primary and essential differences in the 
generative organs. Darwin explained these differences by another 
kind of selective process, namely sexual selection. He pointed out 
that there is competition in courtship as well as in the struggle for 
existence, that the successful males are those which conquer their 
rivals by force, or which please the females best by their beauty of 
appearance or melodiousness of song. Now whether this is true 
or not, and there is certainly a great deal of truth in it, it is not 
sufficient to explain all the facts. In the first place it does not 
explain why the peculiarities of males do not begin to develop until 
the generative organs become functionally mature. If selection by 
the female were the principal factor, an earlier development would be 
an advantage. A male bird, for example, that already had its special 
plumage fully developed when he first became mature, would defeat 
those in which it had only just begun to develop, and consequently 
early development of the special plumage would soon become uni- 
versal. The only way to meet this objection is to maintain that 
the young males find an advantage in being inconspicuous like the 
females, because they thereby escape their enemies, or that they 
obtain some other benefit in the struggle for existence by the re- 
tardation of the development of their secondary sexual characters. 
But when we study the matter without prejudice we find that the 
sexual peculiarities are associated with special habits and conditions, 
which do not come into force until maturity is attained, and we 
have reason to infer that the necessary modifications only occurred 
in connection with these habits and conditions. 
As it is usually the male bird which is stronger, more active, 
and more adorned, some biologists have concluded that the whole 
constitution of the male is naturally more inclined to active 
physiological changes, that of the female more to simple vegetative 
erowth, But there are plenty of cases to show that no secondary _ 
characters are invariably associated with the male sex rather than with 
the female; the evidence indicates that the characters are related to 
particular conditions and habits. In some species the usual differ- 
ences between the sexes are reversed, the male is inconspicuous and 
resembles the young female, while the adult female has peculiar 
characters. In these species we find that the usual habits are also 
reversed, . 
