than from his own. Certain butterflies are polymorphic, and not only
do the males and females of the same species differ, but the females
present two distinct forms, one of which as a general rule mimics the
outward appearance of a distant but highly valued species; yet the
males will pair only with the females of their own kind, and not with
the strangers, though these may be very likely much more like the
males themselves. Among the insect species of the strepsiptera, the
female is a shapeless worm which lives its whole life long in the
hind body of a wasp; its head, which is of the shape of a lentil,
protrudes between two of the belly rings of the wasp, the rest of the
body being inside. The male, which only lives for a few hours, and
resembles a moth, nevertheless recognises his mate in spite of these
adverse circumstances, and fecundates her.

Before any experience of parturition, the knowledge that it is
approaching drives all mammals into solitude, and bids them prepare a
nest for their young in a hole or in some other place of shelter.
The bird builds her nest as soon as she feels the eggs coming to
maturity within her. Snails, land-crabs, tree-frogs, and toads, all
of them ordinarily dwellers upon land, now betake themselves to the
water; sea-tortoises go on shore, and many saltwater fishes come up
into the rivers in order to lay their eggs where they can alone find
the requisites for their development. Insects lay their eggs in the
most varied kinds of situations,--in sand, on leaves, under the hides
and horny substances of other animals; they often select the spot
where the larva will be able most readily to find its future
sustenance, as in autumn upon the trees that will open first in the
coming spring, or in spring upon the blossoms that will first bear
fruit in autumn, or in the insides of those caterpillars which will
soonest as chrysalides provide the parasitic larva at once with food