56 MR. R. GURNEY ON THE LIFE HISTORY OF THE CLADOCERA. 
two years show little difference, and the curves closely 
correspond (diagram i). There is one difference which is 
very striking, namely, that the autumn maximum in 1904 
comes a month later than in the previous year. As a matter 
of fact there are, as it were, two maxima in 1904, the one in 
October corresponding to, but being less than that of 1903. 
But, instead of a fall at once ensuing, the number of individuals 
sexual continued to increase in November. It does not seem 
possible to assign temperature as a cause of this difference, 
though one would be inclined to assume that it was due to 
more favourable conditions of existence prevailing. There 
must be other factors, the influence of which require study, 
such as sunlight, the composition of the water, and the abun- 
dance of vegetation. 
The only previous attempt made in England to study the 
periodicity of the Cladocera in any given area is that of 
Mr. Scourfield (1898*), and as his results differ in some respects 
from my own a few comparisons may be of interest. 
His results are based on 414 collections extending over 
a period of seven years, and made in and around Epping 
Forest. He found that the Cladocera reached their maximum 
number of species in September, and their minimum in 
January, “ with a marked reduction in August.” In Norfolk 
also the maximum is reached in September, but the minimum 
occurs in February, and there is no fall in August. Here are 
the numbers of species for the two years combined : — 
Jan. Feb. March April May June July Aug. Sept. Oct. Nov. Dec. 
25 21 26 33 40 ' 42 45 47 53 51 41 31 
These differences are probably attributable to the locality ; 
since Mr. Scourfield dealt only with ponds, which would be 
more affected by heat in summer and cold in winter than 
the broads are. Otherwise his results agree very closely 
with my own, though there is evidently a more marked 
spring sexual period in the Epping district than in Norfolk — 
a difference also probably connected with the size of the 
ponds. 
Whether external conditions can directly influence sexual 
reproduction is a question that requires further investigation. 
* ‘Essex Naturalist,’ vol. x. 1S98. 
