MR. R. GURNEY ON THE LIFE HISTORY OF THE CLADOCERA. 49 
As I have said, sexual reproduction takes the place of 
agamic reproduction at certain seasons of the year. Generally 
speaking, this sexual phase occurs in the autumn, reaching 
its climax in October or November. Throughout the summer 
males and ephippial females of certain species can be found 
occasionally ; in September a large number are sexual, while 
in October the majority of the commoner species are producing 
resting eggs. The summer -forms begin to disappear in 
November, and sexual reproduction declines, either at the 
beginning or at the end of the month. By December very 
few species are sexual. 
It must not be supposed that every individual produces 
resting eggs ; in many species the majority remain partheno- 
genetic — particularly among the Lynceidae. It is only those 
species which are destined to die out in the winter which 
wholly or in large part change their mode of reproduction. 
Consequently, the abundance of males varies with the species. 
In some species, such as Sida crystallina, they are about 
equal in number to the females, and even exceed them at 
the end of the season, since the females appear to be rather 
more sensitive to cold. In other species, such as Daphnia 
longispina, they are very rare in comparison to the whole 
number of females, though probably they are in tolerably 
equal proportion to the ephippial females. 
But besides the sexual period in autumn there is also 
another, less intense, in early summer. This first period is 
not very clearly marked, but it may be distinguished in the 
table and diagram No. i. The first gives the whole list of 
species and the number of times each has been recorded as 
sexual during the two years. The other gives the total 
number of records expressed, in the form of a curve, as 
percentages of the total number of collections in each month. 
Since exceptional individuals of a species may be found 
sexual when the species as a whole has not really entered on 
a sexual phase, it is best to tabulate, not the number of species, 
but the whole number of records as indicating the intensity 
of the phase for all species together. 
Looking at the diagram, a sharp rise may be seen in the curve 
VOL. VIII. E 
