654 ZOOLOGY. 



it will be remembered that tlie egg develops into an animal 

 by cell-division, so that fundamentally parthenogenesis is. 

 due to cell-division, the fundamental mode of growth ;. 

 hence, normal growth and parthenogenesis are but extremes- 

 of a single series. In this connection, it will be remembered 

 that all the Protozoa reproduce by simple cell-division, 

 that among them the sexes are not differentiated, that they 

 do not reproduce by fertilized eggs ; hence, so to speak, 

 among Protozoa parthenogenesis is the normal mode of re- 

 production ; and when it exists in higher animals it may 

 possibly be a survival of the usual protozoan means of 

 stocking the world with unicellular organisms, with whicli 

 we know the waters teem. And this leads us to the teleol- 

 ogy or explanation of the cause why parthenogenesis has sur- 

 vived here and there in the world of lower organizations ; 

 it is plainly, when we look at the millions of Aphides, of 

 bark-lice, the hundreds of thousands inmates of ant-hills, 

 and bee-hives, for the purpose of bringing immediately 

 into existence great numbers of individuals, thus ensuring, 

 the success in life of certain species exposed to great vicis- 

 situdes in the struggle for existence. That this unusual 

 mode of reproduction is all-important for the maintenance 

 of the existence of most of the parasitic worms, is abundantly 

 proved when we consider the strange events which make up 

 the sum total of a fluke or tape-worm's biography. With- 

 out this faculty of the comparatively sudden production of 

 large numbers of young by other than the slow, limited 

 process of ovulation, the species would be stricken of! the- 

 roll of animal life. 



Dimorphism and Polymorphism. Involving the produc- 

 tion of young among many-celled animals (Metazoa) by what 

 is fundamentally a budding process, we have two sorts of 

 individuals. "When the organism is high or specialized 

 enough to lay eggs which must be fertilized, we have a 

 differentiation of the animal into two sexes, male and fe- 

 male. Reproduction by budding involves the differentia- 

 tion of the animal form into three kinds of individuals 

 i.e., males, females, and asexual individuals, among insects. 

 often called workers or neuters. These have usually, as in. 



