MR. R. GURNEY ON THE LIFE HISTORY OF THE CLADOCERA. 53 
mainly those which live habitually or commonly in ponds or 
ditches exposed to the chance of drying up or to extremes of 
heat — for example Daphnia pulez. Bosmina longirostris is 
an apparent exception, but it is a species which appears to 
die out to some extent in summer. The genus Bosmina is 
one of probably Arctic origin, and it is possible that, like 
Cyclops strenuus and, to some extent, C anthocamptus 
staphylinns, it is not able to bear the high temperature of 
summer. 
As a matter of fact, here in Norfolk, conditions seem to be 
exceptionally favourable all the year round. In Germany 
apparently many species die out completely in winter which 
here remain abundant — e.g., Daphnia longispina. Apparently 
as a direct result of this I find many species to be either 
acyclic here, or very rarely to show sexual individuals. For 
example, I have never seen an ephippial female or a male of 
any species of Macrothricidae ; one of the commonest of the 
Lynceidae, Eurycercus lamellatus was never found sexual 
during 1903 and only a few times in 1904. 
If, as I suppose, their whole economy is affected by their 
permanent or average surroundings, it becomes a question 
whether it may also be affected by their changing surroundings 
— by weather for instance. 
The weather of 1903 differed considerably from that of 
1904, and diagram No. 2 shows the mean temperature of each 
month of those years in the form of a curve. In table No. 2 
I have selected certain of the species which are specially 
summer forms and of which the records are sufficiently full 
to be reliable. Table No. 1 gives the whole list of species 
for the two years for comparison. It will be seen that in 
1903 nine of the summer forms appeared earlier than they 
did in 1904, that is to say, thougl^in'one or two cases they 
were first recorded in the same month in each year, still 
there were differences of ten'days or so. Only one species, 
Ceriodaphnia megalops, appeared earlier in 1904 than in 1903. 
It is probable that the resting eggs require a certain 
minimum of heat, different perhaps for different species, 
before they can develop, and that this minimum was reached 
