AND MORPHOLOGY OF APHIS. 



219 



Finally, the produced zooid may be incapable of development into an independent 

 organism, unless it conjugate with another zooid. In this case we have sexual reproduc- 

 tion, or gamogenesia. 



The natural character of this classification of the various modes of development is 

 manifest when it is thro^^^l into a tabular form : — 



Development. ■< 



Continuous 



Discontinuous 

 (Gemmation with 

 fission). 



Agamogenesis. 



Garaogenesis. 



Growth. 

 Metamorphosis. 

 Gemmation without fission. 

 f Metagenesis. 



I Parthenogenesis. 



Whatever hypothesis we may entertain with respect to the nature of these processes, 

 and however we may think fit to conceive the nature of the "individual," I think it 

 must be admitted, that all the phenomena of development in the animal kingdom (and I 

 would venture to add, in the vegetable kingdom also) fall under one or other of these 

 heads. 



Furthermore, all these modes of development pass into one another. Growth and 

 metamorphosis are combined in all animals. Gemmation, so long as the gemma continues 

 attached, is but a peculiar kind of growth and metamorphosis. From the fixed bud to the 

 separate one, we have aU gradations ; and fission is little more than a peculiar mode of 

 budding. 



Free gemmation is " metagenesis " when the bud is not developed within the homologues 

 of the sexual reproductive organs; it becomes "parthenogenesis " when the bud is deve- 

 loped within such organs ; finally, when the free bud requires conjugation with another 

 free bud for its development, we have gamogenesis, or sexual reproduction : but cases 

 such as those of Daphnia and Apis show that the histological element, which is at one 

 time agamogenetic, may at another be gamogenetic. 



Time was when the difficulty of the physiologist lay in understanding reproduction with- 

 out the sexual process. At the present day, it seems to me that the problem is reversed, 

 and that the question before us is, why is sexual union necessary ? Far from seeking for 

 an explanation of the phenomena of gemmation in the transmitted influence of the sper- 

 matozoon, the philosopher acquainted with the existing state of science will seek, in the 

 laws which govern gemmation, for an explanation of the spermatic influence. 



VOL. XXII. 



26 



