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 

 growth. 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. 



