916 
Proceedings of the Royal Society 
cycles of asexual reproduction, we find hints of sexual dimorphism 
and fertilisation. Both among Badiolarians and Infusorians the 
conjugating elements may differ in size ; in the Volvocinece especi- 
ally macro- and microspores occur, which may conjugate with one 
another, though the process is usually restricted to the latter. 
The almost mechanical flowing together of exhausted cells, as 
illustrated in plasmodia, is connected through the known surviving 
cases of multiple conjugation with normal conjugation, and we 
have just seen how gradually the dimorphism appears which marks 
the transition from conjugation to fertilisation, and makes the 
latter indispensable. Enough has been said to indicate this 
extremely gradual differentiation of asexual into sexual reproduction. 
The very gentleness of the gradation leads one to regard the two 
processes as analogous responses to the same physiological neces- 
sities. The same disturbance of the balance between anabolism and 
katabolism which results in the occurrence of asexual reproduction, 
when continuous growth or continuous cell-division was no longer 
possible, leads, in more developed forms, to the separation of the 
dimorphic and mutually dependent elements of sexual reproduction. 
As asexual reproduction occurs at the limit of growth, so a check 
to the asexual process involves the appearance of the sexual, which 
is thus still further associated with katabolic preponderance. 
The familiar history of the Aphides may serve as an illustration. 
During the summer months, with favourable temperature and 
abundant food, the Aphides produce parthenogenetically, generation 
after generation of females. The advent of autumn, with its 
attendant cold and scarcity of food, brings about the birth of males, 
and the consequent recurrence of strictly sexual reproduction. In 
artificial environment, equivalent to a perpetual summer, the asexual 
process may he prolonged for years, while a lowering of the 
temperature and diminution of the food at once reintroduce sexual 
reproduction. The occurrence of the latter is, in other words, 
associated with the setting in of conditions which make for kata- 
holism. 
This opposition between nutrition and reproduction, which, after 
life and death, is the most obvious antithesis in nature, is 
often taken advantage of for practical purposes. The removal 
of the reproductive organs, in flowers, for instance, increases the 
