10
THE ESSEX NATURALIST.
temperate and sub-arctic regions is for the individuals which
hatch out in the spring or early summer from the winter- or
resting-eggs, and which are always females, to produce a suc-
cession of broods consisting wholly of females. These in their
turn and their progeny for some months produce only females, but
towards the autumn males appear and some or all of the females,
the so-called "ephippial" females, acquire the power of produc-
ing the winter- or resting-eggs already alluded to which, after
fertilisation, remain quiescent until the next year. There are-
many variations of this general cycle. In some species males
and "ephippial" females appear quite early in the year, although
it is usually only among the members of those species inhabiting
small pieces of water liable to being dried up in the summer
that this occurs. On the other hand, some species in some
situations seem to be able to go on producing parthenogeneti-
cally all through the year, and possibly in some cases indefinitely.
The Ostracoda also very commonly reproduce by parthenogenesis,
and here again the process appears to go on indefinitely in some
cases, no males of certain species ever having been seen. On the
other hand the Copepoda do not appear to be parthenogenetic
as a rule, although it is probable that some species at least are
capable of this kind of reproduction.
There are three points of interest in connection with partheno-
genesis to which attention may be called. In the first place,
is it of any benefit to those species and groups of species in
which it occurs? This can probably be answered in the affirma-
tive, for it is evident that much more rapid reproduction can
take place if all the representatives of the species are actively
producing young and, moreover, the risks of non-fertilisation
are eliminated. That this method of reproduction, however, is
not wholly advantageous is fairly certain, as otherwise it would
occur more frequently. A second point of interest attaching
to parthenogenesis is in regard to the determination of sex.
The general opinion now is that sex, primarily at any rate, is
dependent upon the presence or absence of an unpaired chromo-
some in the nuclei of the cells of the body, or upon the presence
of either a similar pair or a dissimilar pair of chromosomes.
This, however, is known not to be the whole story in many cases,
and indeed it can hardly be so in the parthenogenetic Cladocera.
Apparently in this Order we have to account for (1) females