INTRODUCTION. xlix 



workers lay eggs which result in the production of males, while the fertilized eggs laid 

 by the female ant or queen bee produce females or workers. 



Taking all these cases together, parthenogenesis is seen to be due to budding, or 

 cell-division or multiplication. Now 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 partheno- 

 genesis 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 

 dif ferentiate d, that they do not.reproduce by fertilized eggs ; hence, so to speaETamong 

 Protozoa, parthenogenesis is the normal mode of reproduction ; 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 teleology 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 vicissitudes in the struggle for existence. That this unusual mode of reproduc- 

 tion 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. Without this faculty of the compara- 

 tively sudden production of large numbers of young by other than the slow, limited 

 process of ovulation, the species would be stricken ofE the roll of animal life. 



Dimorphism and Poltmoephism. 



Involving the production 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 female. Reproduction by bud- 

 ding involves the differentiation 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 ants and bees, a distinct form, so as to be 

 readily recognized at first sight. Among the Coelenterata and worms the forms repro- 

 ducing by parthenogenesis are usually larval or immature, as if they were prematurely 

 hurried into existence, and their reproductive organs had been elaborated in advance 

 of other systems of organs, for the hasty, sudden production, so to speak, of large 

 numbers of individuals like themselves. 



In insects, dimorphism is intimately connected with agamic reproduction. Thus 

 the summer wingless, asexual Aphis and the perfect winged autumnal Aphis may be 

 called dimorphic forms. The perfect female may assume two forms, so much so as to 

 be mistaken for two distinct species. Thus, an oak gall-fly ( Cynips quercus-spongificd) 

 occurs in male and female broods in the spring, while the autumnal brood of females 

 was described originally as a separate species under the name G. atiiculata. Walsh 

 considered the two sets of females as dimorphic forms, and that Cynips aciculata lays 

 eggs which produce G. quercus-spongifica. Among butterflies, dimorphism occurs. 

 Papilio memnon has two kinds of females, one being tailless, like the tailless male, 

 while Papilio pammon is polymorphic, there being three kinds of females besides 

 the male. 



