THE LIFE-CYCLE OP MOIXA EECTIROSTEIS. 
517 
chance chosen for these experiments.^ The suppression of the 
sexual forms must therefore be certainly attributed to the 
condition of high temperature and isolation of the parents. 
In line 5 eight successive generations^ containing 413 indi- 
viduals,, were propagated without the appearance of sexual 
forms. 
The effect of isolation is also seen in the fact that the isolated 
parents at room temperature and in the cold chamber gave a 
smaller proportion of males than when the parents were 
crowded under the same conditions, viz. 19T per cent, and 
5'4 per cent, males as against 52'3 per cent, and 42'5 per cent. 
The effect of tempei-ature by itself is seen in the fact that 
when the jDai’euts were isolated in the incubator they gave 
0 per cent, males, while isolation at room temperature gave 
19T per cent, males; also crowding in the incubator gave 30'3 
per cent, males, while crowding at room temperature gave 
52'3 per cent. 
The effect of low temperature, i. e. in the cold chamber at 
about 5°C., gave actually a lower percentage of males than 
at room temperature both when the parents were isolated and 
when they were crowded. 'I’he numbers of broods and of 
individuals produced uuder these conditions were small, as it 
was found that the extremely low temperature inhibited growth 
and reproduction. It might be expected a priori that since 
more males are produced at room temperature tliau in the incu- 
bator, a still gi-eater proportion of males would be produced 
in the cold, but it appears that the factor of extreme cold intro- 
duces another element — possibl}' that of very slow growth — 
into the conditions, which acts in an opposite direction. 
The influence of i.solation and of crowding can be shown by 
another method of arranging the results. In Table II are given 
> Prof. R. C. Pumiett, in liis work on “ Hydatina,” lias suggested that 
such female-producing lines exist. Owing to the way in which our ex- 
periments were performed, viz. by subjecting the individual members of 
each brood to different conditions, it is impossible to account for our 
results in this way. We are, however, not in a position to deny that it 
may be possible to segi-egate out a pure female-2>roducing line in Moina 
in accordance with Punnett's suggestion. 
