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 gamogenesis. 
The natural character of this classification of the various modes of development is 
manifest when it is thrown into a tabular form :— 
Growth. 
Continuous 1 Metamorphosis. 
Gemmation without fission. 
Development. * . [Metagenesis. 
Agamogenesis. 
Parthenogenesis. 
Discontinuous $ 
(Gemmation with 
fission). | Gamogenesis. 
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 all 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. ve 
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, > 
