plant, from which it departs by morphological and physiological
variations suited to a more energized form of life; and the female may
be regarded as the animal norm from which the male departs by further
morphological variations. It is now well known that variations are
more frequent and marked in males than in females. Among the lower
forms, in which activity is more directly determined mechanically
by the stimuli of heat, light, and chemical attraction, and where
in general the food and light are evenly distributed through the
medium in which life exists, and where the limits of variation
are consequently small, the constitutional nutritive tendency of
the female manifests itself in size. Among many Cephalopoda and
Cirripedia, and among certain of the Articulata, the female is larger
than the male. Female spiders, bees, wasps, hornets, and butterflies
are larger than the males, and the difference is noticeable even in
the larval stage. So considerable is the difference in size between
the male and female cocoons of the silk-moth that in France they are
separated by a particular mode of weighing.[24] The same superiority
of the female is found among fishes and reptiles; and this relation,
wherever it occurs, may be associated with a habit of life in which
food conditions are simple and stimuli mandatory. As we rise in the
scale toward backboned and warm-blooded animals, the males become
larger in size; and this reversal of relation, like the development
of offensive and defensive weapons, is due to the superior variational
tendency of the male, resulting in characters which persist in the
species wherever they prove of life-saving advantage.[25]

The superior activity and variability of the male among lower forms
has been pointed out in great detail by Darwin and confirmed by
others.