654 ZOOLOGY. 



it will be remembered that the 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 which 

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

 ogy or explanation of the c.iuse 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 i:p 

 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 off 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 



