THE LIFE-CYCLE OF MOINA IIECTIEOSTRIS. 
515 
which males were being produced; so that it appears tliat the 
conditions which call forth the production of males are the 
same as those which determine the production of ephippial as 
opposed to parthenogenetic females. 
A good deal of confusion has arisen in regard to the life- 
cycle of Cladocera by the tacit assumption that the conditions 
controlling the production of males are sex-determining con- 
ditions — that alterations in the food supply^ temperature, etc., 
condition the production of males as opposed to females. 
This is an altogether mistaken view. Alterations in the 
external conditions do not alter the sex-ratio, except in so far 
as they may change the method of I’eproduction from that by 
means of parthenogenetic females to that by means of sexual 
males and females. For instance, in our own experiments 
isolation in the warmth led to the entire suppression, not only 
of males, but also of sexual females, while crowding at room 
temperature or in the cold led to the increase of males and 
ephippial females as opposed to parthenogenetic females. 
Thus, what is really shown to be influenced by the external 
conditions is not the sex-ratio, but the production or suppres- 
sion of the sexual forms of both sexes. 
The same stricture applies to all the experimental work on 
the life-history of the Cladocera, which is sometimes loosely 
termed work on sex-determination, but which is really 
calculated to throw light, not on the determination of sex, 
but on the alternation of asexual with the sexual mode of 
reproduction. Since in our experiments the same conditions 
called forth the production of males and sexual females, we 
can only conclude that these conditions were not specific sex- 
determining conditions, but simply determined that sexual 
males and females should be produced as opposed to partheno- 
genetic females. It is, of course, possible that external con- 
ditions, besides influencing the occurrence of sexual as 
opposed to asexual reproduction, may also influence the pro- 
portions of males to sexual females in any brood, but our 
experiments do not throw any light on this point, and the 
experiments of other observers appear to be beset by the 
