52 MR. R. GURNEY ON THE LIFE HISTORY OF THE CLADOCERA.. 
for 1903 in June, followed by a fall in July and August, 
before the great rise showing the chief sexual period in October. 
There is a break in the records for July and August of 1904, 
but the records for June and September seem to indicate 
a similar course in July and August, though probably the 
maximum fell actually in July in that year and not in June. 
Generally speaking, those species that are sexual in summer 
are also sexual in autumn, and these are called Polycyclic 
species, since they have more than one cycle in the year. 
That is to say, the individuals of the first, or spring cycle, 
lay resting eggs which hatch in the late summer and give rise 
to the individuals of the second, or autumn cycle. For 
example : Ceriodaphnia pulchella. Those that have but 
a single cycle in the year are called monocyclic, e.g., Sida 
crystallina. Those that are always found parthenogenetic 
are acyclic. As Weismann* surmised, there are probably no 
acyclic species , but only acyclic colonies of cyclic species. 
The category into which any species falls varies and depends 
upon its surrounding conditions. In the Arctic regions 
agamic reproduction plays a relatively unimportant part, 
and the number of generations between the individuals 
hatched from the resting eggs and the sexual brood is generally 
few. In the true Arctic conditions all species, with a few 
exceptions, are Monocyclic, owing to the short space of time 
during which the water is free from ice (Ekman 1904!). 
Now, inasmuch as the production of resting eggs is a measure 
for the protection of the species from adverse conditions, one 
would expect that where such adverse conditions do not 
constantly recur, parthenogenesis would be retained as being 
the most rapid mode of multiplication. Extremes of heat 
and cold and the drying up of water are the usual causes of 
destruction. As the Norfolk Broads do not dry up and are 
deep enough not to get over heated, it is only to be expected 
that the first cycle, which seems to be more pronounced in 
other countries, should here be rather indistinct. The 
species which are recorded as sexual in early summer are 
* Zeits. Wiss. Zool., vol. xxxiii., 1883. 
t Zool. Jahrb. , Bd. xxi., 1904. 
