168 REPRESENTATION OF THE 



find tliat among animals generally the parts ministering to 

 the generative function have regularly recurring periods of 

 increased action, dependent on the development or gemma- 

 tion, as it were, in their substance, of swarms of cells, bear- 

 ing the reproductive corpuscules, and comparable in a de- 

 gree to the piles of medusoids budded off from the polype 

 stock. During the intervals the function is commonly en- 

 tirely suppressed, and the parts, so largely developed dur- 

 ing the period of action, are in some cases not to be dis- 

 tinguished at all. Even in species characterized apparently 

 by continuous fertility, the phenomena of ovulation in the 

 female, which are always more or less periodic, are gener- 

 ally admitted to indicate corresponding differences in the 

 proclivity for conception. 



Another argument in the same direction, and one pro- 

 bably of greater force, may be drawn from the lateness of 

 the development of the reproductive organs. In some 

 species they would seem actually not to be formed at all 

 till the breeding period arrives. This has been already 

 noticed in the case of the Distoma, in which the appear- 

 ance of these organs constitutes almost as marked an 

 epoch, as the previous budding off of the Cercariw from the 

 interior of their gregariniform matrix. In some instances 

 it is equally dependent on the transference of the parasite 

 to an animal of different species from its former victim. 

 The Cercaria which has quitted the snail, on its discharge 

 from its parent sac, for the body of an aquatic insect, may 

 in this position complete its larval development, and yet 

 may not mature its sexual organs, till, by its new host 

 falling a prey to the voracity of some bird, it is transferred 

 to the intestine of a warm-blooded animal. 



Another peculiarity in the development of the sexual 

 organs may be noticed in this connection — viz., that among 

 insects which live in communities — such as Bees, Ants, and 

 Termites — these organs are never perfectly developed at all, 



