SOME BIOLOGICAL PROBLEMS. 9
same way the fourth figure (lower to right) shows that an outline
very similar to that characteristic of the genus Ceriodaphnia can
be obtained if the outline of D. pulex be transferred to a network
with the horizontal lines placed at diminishing distances apart
as measured from the centre line, the vertical lines remaining as
in the first figure. I think that these figures add some confir-
mation to the idea that, in the main, organic variation cannot
be a mere alteration of this or that detail, but that it involves
a close correlation of all associated parts. Just as a deformation
of any part of a network of co-ordinates leads to correlated
alterations, theoretically at least, in every other part, so an
alteration in any part of an organism gives rise to correlated
modifications in many, if not in all, other parts. At bottom,
no doubt, such correlation is just another illustration of the
principle that wholes are something more than the mere sum of
their parts, christened by General Smuts "Holism" in his
book on "Holism and Evolution."
Turning now to the physiological problems we may consider
first the remarkable phenomenon of parthenogenesis and inci-
dentally the closely related subject of the determination of sex.
The word parthenogenesis is, I need scarcely explain, used to
denote the state of affairs in which the female of a species is able
to produce offspring without the aid of a male. Such power is
not known to exist naturally among any of the vertebrates,
although frogs have been reared artificially from unfertilised
eggs. But among invertebrate animals parthenogenesis is a
naturally occurring phenomenon in several groups, such as
Rotifers, Insects and Crustacea. The cases most widely known
perhaps are those of the Aphides, or green-flies, which go through
many generations in the summer without the appearance of a
male, and of the drone-bees which are produced from unfertilised
eggs. Parthenogenesis also occurs in plants, as in the dandelion,
but it does not seem to be common. There is every reason
to suppose that parthenogenesis is a secondary condition
occurring sporadically among groups of organisms otherwise
normally bisexual and not a throw-back to an originally asexual
condition. This is sufficiently indicated perhaps by the different
degrees of its manifestation in closely allied forms, a point which
is very well illustrated by the Entomostraca.
Among the Cladocera, for instance, the general rule in