does not
necessarily rely on the blood of warm-blooded animals. The mouth
parts of the male are so different from those of the female that
it is probable that, if it feeds at all, it obtains its food in
quite a different manner from the female. They are often
observed sipping at drops of water, and in one instance a
fondness for molasses has been recorded."[29]

We find many examples of such structural modifications acquired for
the purpose of adapting the sexes to different modes of life. Darwin
notes that the females of certain flies are blood-suckers, whilst the
males, living on flowers, have mouths destitute of mandibles.[30] The
females are carnivorous, the males herbivorous. It would be easy to
bring forward many further examples among the invertebrates in which
the differences between the sexes indicates very clearly the
persistence of female superiority. But for these I must refer the
reader to the works of Darwin and other entomologists, and to the many
interesting cases given by Professor Lester Ward. There are, it is
true, exceptions, but these may be explained by the conditions under
which the species live.

Even when we ascend the scale to back-boned animals, cases are not
wanting in which the early superiority in size of the female remains
unaltered. The smallest known vertebrate, _Heterandria formosa_, has
females very considerably larger than the males.[31] Among fishes the
males are commonly smaller than the females, who are also, as a rule,
considerably more numerous.[32] This is a fact that fishermen are well
aware of. I may mention, as an example, that on one occasion when my
husband and I caught twenty-five trout in a mountain lake in Wales
there were only two males among them. It is curious to find that any
care of offspring that is evident among fishes is usually paternal.
This furnishes another instance of the truth so necessary to learn
that the sex-relationships may assume almost any form to suit the
varying conditions of life.

There are some mammals among whom the sexes do not differ appreciably
in size and strength, and very little or not at all, in coloration and
ornament. Such is the case with nearly all the great family of
rodents. It is also the case with the Erinaceidae, or at least with its
typical sub-family of hedgehogs.[33] Even among birds, where the sex
instincts have attained to their highest and most aesthetic expression,
we find some large families-