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 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 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 
