354 GENERAL PRINCIPLES 



are regularly developed, male and female. The difference be- 

 tween the two is often indistinguishable except in the gonads, 

 and it may require dissection to determine the sex. In other 

 cases the gonads are visible through the transparent wall of the 

 body, and a difference in color of those organs often distinguishes 

 the sexes (jelly fish, some worms, etc.). More frequently there 

 are secondary sexual differences, such as accessory sexual 

 organs, egg-laying apparatus, or copulating organs or structures 

 which are more remotely or not at all connected with the function 

 of reproduction. The female is very generally larger than the 

 male, a fact which is probably to be connected with her greater 

 trophic functions. This is notably the case among insects. 

 In a few remarkable instances the male is minute, compared 

 with the female, and may even be attached to her as a parasite. 

 (Ex.: Barnacles, Sacculina, Oedogonium.) Among Mammals 

 the males often fight with each other for the possession of the 

 females, and this has resulted in a greater development in size 

 and strength of the males. The male of the fur seal is four 

 times larger than the female. Among Birds the difference be- 

 tween the sexes is most conspicuous with regard to coloration 

 and song, in which the males usually far excel the females. 

 (See p. 423.) Among butterflies there are often remarkable 

 differences in coloration between the sexes, and in a number of 

 Insects the male is winged while the female is without wings 

 (glowworm and Hibernia moth). Among plants, sexual 

 dimorphism is usually evident only in the accessory reproductive 

 organs (flowers). 



740. Polymorphism. — There is also a manifolding of form 

 types which has no direct relation to sex. It is best developed 

 in lower forms, especially those which are colonial. Among the 

 Hydrozoa there may be as many as four or five types of indi- 

 viduals. These may be classed as the trophic or feeding polyps, 

 the budding polyps, the protective polyps, and the sexual me- 

 dusae. In the Siphonophores there are, in addition, the swim- 



