THE EVOLUTION OF SEX. 



CHAPTER I. 



The Sexes and Sexual Selection. 



THAT all higher animals are represented by distinct male 

 and female forms, is one of the most patent facts of 

 observation, striking enough in many a beast and bird to catch 

 any eye, and familiarly expressed in not a few popular names 

 which contrast the two sexes. In lower animals, the contrast, 

 and indeed the separateness, of the sexes often disappears; yet 

 even naturalists have sometimes mistaken for different species, 

 what were afterwards recognised to be but the male and female 

 of a single form. 



§ I. Primary and Secondary Chai'aders. — When we pass 

 from this commonplace of observation and experience to inquire 

 more precisely into the differences between the sexes, we speedily 

 recognise that these are of very different degrees. In some cases 

 no marked differences whatever are recognisable ; thus a male 

 star-fish or sea-urchin looks exactly like the female, and a care- 

 ful examination of the essential reproductive organs is requisite 

 to determine whether these respectively produce male elements 

 or eggs. In other cases, for instance in most reptiles, no 

 external differences are at all striking, but the aspect of the 

 internal organs, both essential and auxiliary to reproduction, at 

 once settles the question. In a great number of cases, again, 

 the sexes resemble one another closely, but each has certain 

 minor structural features at once decisive as to its respective 

 maleness or femaleness. Thus in the males there are 

 frequently prominent organs used in sexual union, while the 

 peculiar functions of the females are indicated in the special 

 egg-laying or young-feeding organs. All such characters, 



