518 
G. H. GEOSVENOR AND GEOFFREY SMITH. 
the proportions of males and females produced at room tem- 
perature according to the number of parents present in the 
vessel. The number of parents kept in the same vessel varied 
from one to thirty-four. It cannot be said that the proportion 
of males to females steadily rises as the degree of crowding of 
the parents increases, as the numbers fluctuate rather erratic- 
ally, but by grouping the numbers together in various ways it 
can be shown that on the whole the proportion of males does 
increase with an increased intensity of crowding. For instance, 
to take the largest grouping, when one parent was present 
19‘1 per cent, males were produced, when two to seven parents 
37'3 per cent, males, when eight to thirteen parents 60'9 per 
cent, males, and when fourteen to thirty-four G5’2 per cent, 
males. We may take it, therefore, as established that the 
two factors of crowding and temperature profoundly influence 
the production of the sexual forms, and that by isolation of 
the parents at a temperature of about 28° C. it is possible to 
suppress entirely this production. 
The inquiry as to how these factors of crowding and 
temperature brought about this effect was found to be beset 
with great difficnlties. It seemed probable at first that the 
effect of crowding was due to the accumulation of excretoiy 
matter in the glasses, but experiments in which isolated 
females were placed in culture-water which had previously 
contained numerous individuals of Moina gave negative 
results, the isolated females in the incubator in such culture- 
water producing invariably females. Culture-water which 
had contained great numbers of Moina was also evaporated 
down to dryness and the residue dissolved in fresh culture- 
water, but again with negative results. It is, of course, 
possible that the excretory matter is of an unstable character, 
and quickly oxidised or destroyed as soon as it is formed. 
There is, however, another possibility, viz. that the effect of 
crowding is to lessen the food supply for each individual. 
The food of Moina consists of the organisms in the infusion, 
but it is uncertain whether the Cladoceran canfeedindifferently 
on all the bacteria and infusoria found in such infusions, or 
