MR. R. GURNEY ON THE LIFE HISTORY OF THE CLADOCERA. 57 
Weismann, to whom we owe most of our knowledge of this 
Cyclic reproduction, concluded that the time of appearance 
of sexual individuals is not determined by external conditions, 
but by some internal factor. He found that in all the species 
which he closely investigated these individuals appeared at 
certain determined generations. The first generation hatched 
from the resting eggs is always parthenogenetic, but males 
and ephippial females may appear in the second generation, 
as in Moina rectirostris, or in any one of the following 
generations according to the species. The resting eggs 
deposited by the individuals of the first cycle lie dormant for 
a time and give rise to the second autumn cycle. Such a 
definite law, namely, that sexual individuals appear at a 
determined generation only, unaffected by external conditions, 
does not, I think, express the whole truth. In the Arctic 
regions the cycle is very much shorter than in the south 
(Ekman, 1904) ; in fact, several species there have only 
a single parthenogenetic generation, and it is fair to assume, 
in the absence of proof to the contrary, that the length of 
the cycle is, as it were, a plastic thing moulded by the dim te. 
The fate of individuals which survive the winter is of 
particular interest in this connection. According to Weismann 
their progeny should continue to reproduce parthenogeneti- 
cally till they die out. I have followed the fate of such an 
individual of Daphina longispina. A single female was taken 
on February 6, 1903, and the history of her progeny was 
followed in detail to the seventh generation in August and 
less completely till October 13. Reproduction was at^first 
very slow, so that the individuals of the third generation did 
not produce young till May 10. TheTive individuals^then 
under observation all began on or about that date to produce 
sexual young, and three of them themselves became ephippial. 
For the rest of the time till August males were occasionally 
born and ephippia produced. In collections made at Sutton 
sexual individuals were first found on May 8, and continued 
to occur till the end of July. The correspondence is very 
striking and must be explained as showing the influence of 
temperature, as there could be no question here of a definite 
number of generations. 
