io8 DARWINISM TO-DAY. 



CLASSIFICATION OF THE EXTERNAL SECONDARY SEXUAL CHARACTERS. 



Group A. Characters which are useful to the possessor, or to its 

 young, or have an indirect relation to reproduction. 



Sub-group i. Specialisations in organs which aid in the finding of 

 individuals of the other sex. Examples, the extra-develop- 

 ment of the antennae in many male moths and beetles, the 

 enlarged and divided eyes of certain flies and May-flies, the 

 enlarged tactile feelers of male Daphnias, the larger or better 

 wings of many male insects, the swimming membrane, in the 

 breeding season, on the hind-legs of Molge paradoxa. 



Sub-group 2. Specialisations that aid in mating. Examples, the 

 clasping organs of many male crabs, the hectocotylus of 

 octopuses, the expanded tarsi of many male insects, and in 

 general, the accessory copulatory organs of innumerable vari- 

 ous animal species. 



Sub-group 3. Special size and form of the female due to the extra- 

 development of the ovaries. Examples, in Psychid moths and 

 parasitic Crustaceans. 



Sub-group 4. Differences connected with care of the young. Ex- 

 amples, mammae of female mammals, brood-pouch of mar- 

 supials, brood-sacs of male sea-horses, and brood-cavities in 

 the back of male Pip a (a frog). 



Sub-group 5. Specialisations for defence or offence. Examples, 

 protective coloration of female birds and insects, mimicry 

 by female butterflies, antlers of the stag, strong canines of 

 many male mammals (wild boars, etc.), sting of the female 

 honey-bee, spurs of the cock, greater size and strength of 

 many male mammals and birds. 



Sub-group 6. Differences in coloration which aid in the recogni- 

 tion of the sexes ("recognition marks" of Wallace). 



Sub-group 7. Differences connected with various special habits of 

 life. Examples, the pollen-baskets of the worker honey-bees, 

 the winglessness of male fig-capri tying insects (Blastophaga), 

 the large differences between males and females of certain 

 insects where one sex lives parasitically, the other independ- 

 ently, as the scale insects, the Strepsiptera, etc., the beak 

 differences in the New Zealand bird, Heteralocha acutiros- 

 tris, whose male chisels out the hard wood with a short, 

 broad beak, while the female extracts insect larvae from decay- 

 ing wood by means of a long, curved beak. 



Group B. Exciting organs. These are found almost exclusively in 

 the males only, and serve to indicate the sexual excitement 

 of their possessors, and at the same time to stimulate or excite 



