THE EVOLUTION OF SEX. 



CHAPTER I. 

 THE SEXES AND SEXUAL SELECTION. 



'T'hat all higher animals are represented by distinct male and 

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

 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 recognized to be 

 but the male and female of a single form. 



I. Primary and Secondary Characters. — When we pass 

 from this commonplace of observation and experience to inquire 

 more precisely into the differences between the sexes, we speedily 

 recognize that these are of very different degrees. In some cases no 

 marked differences whatever are recognizable; thus a male starfish 

 or sea-urchin looks exactly like the female, and a careful 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, directly associated with the essential 

 functions of the sexes, are included under the title of primary sexual 

 characters. 



Of less real importance, though often much more striking, are the 

 numerous distinctions in size, color, skin, skeleton, and the like, which 

 often signalize either sex. These are termed secondary sexual 

 characters; for though they will be shown in some cases at least to be 



