THE LIFE-CYCLE OF MOINA RECTIROSTEIS. 
519 
whether they are confined to some particular kind of food. 
If they are omnivorous it is inconceivable that the presence of 
eight or nine individuals in our culture-glasses should cause 
any shortage of food, but if they really confine themselves to 
some particular organism in the infusion as food it is very 
probable that there is a limited supply of this organism, and 
that the presence of several Moina is sufficient to upset the 
balance. From the behaviour of Moina in the culture-medium 
it Avould seem that they are actively hunting some special 
prey, and the peculiarly local occurrence of the various kinds 
of Cladocera points to their being in general dependent on 
special organisms for food. There- can be no doubt that an 
important step forward will be taken when we can settle this 
question and cultivate these Cladocera on relatively pure 
cultures of their appropriate food. When the organisms are 
cultivated ou the mixed and complicated fauna of an infusion, 
as was the case in our experiments, it is impossible to regulate 
the food supply, as thei’e is no means of controlling the rela- 
tive abundance of the particular constituent which alone may 
be serving as food. 
For the present we must be content to say that under 
certain optimum conditions, which can be attained by isola- 
tion and a moderately high temperature, it is possible to 
propagate Moina entirely by parthenogenesis without any 
pi’oduction of sexual forms, but whether this result is due 
to the absence of excretory matters or to an abundant food 
suppl}’’ it is impossible to decide. 
Although our results are incomplete in this direction, yet we 
ai-eable to furnish a definitely negative answer as to the truth 
of Weismann’s contention that the life-cycle is an hereditarily 
fixed process which runs its course independently of external 
conditions. It is, however, by no means certain that the life- 
cycles of other Cladocera are as sensitive to external con- 
ditions as IMoina. It may be found possible to inhibit the 
pi’oduction of sexual forms indefinitely in other species of 
Cladocera, but it seems probable that the artificial production 
of sexual forms in any particular generation of those forms 
