LIMNOLOGICAL PROBLEMS 
413 
that they can no longer be met. The research shows that in Hyalo- 
daphiiia brood- and growth-stages often occur of an extremely high 
and weak structure, but these are not found again in mature animals : 
they occur only when the temperature of the water is at its highest, 
Fig. 58. — Hyalodaphnia cucullata. Seasonal variation in the growth-stages (Fureso). 
During growth the crest in the summer half-year increases so much that it is 
almost as long as the valves ; in the winter half-year the crest does not grow at all. 
and their lifetime seems always very short. The view which I 
have taken of the phenomenon is that the claims of the outer 
conditions laid at this time on the species are so great that each 
individual may well meet them but has 
been forced so near the limits of the 
variability of the species that the individual 
pays for its extreme by sterility. The 7nost 
ejrtreme variations produced hy outer condi- 
tions call thus 7iot be inherited. (Fig. 59.) 
In the beginning of autumn, the great 
increase in the parts of the body which 
have to counteract the rate of sinking 
ceases during growth. Somewhat later, 
generations are produced in which, on 
hatching, the crest is shorter and does not 
increase during growth. Towards the 
winter the species has again the exterior 
which we are accustomed to regard as 
normal. 
In what manner does the individual 
transform itself ? We must restrict ourselves to the following remark. 
There is reason for believing that the outer conditions which produce 
the seasonal variations principally assert themselves, or at any rate 
make their influence felt, just at the period succeeding the new moult. 
If this holds good, then the seasonal variations, i.e. the faculty of the 
Fig. 59. — HyalodaiJhnia cucul- 
lata. Individuals which 
the demands for increased 
floating power have forced 
beyond the limits of elasti- 
city of the species. They 
do not attain to the mature 
stage, and in any case do 
not produce eggs. 
