ants, this plan of double
parenthood, the sexual method, alone is used.

In order that we may the more clearly understand how the mammals
produce their young and nourish them, we shall begin at the lowest
class of the backboned animals and note how the process is there
accomplished. As we pass upward through the kingdom the method
acquires greater complexity. When we finally reach the mammals, what
at first seemed an absolutely new process will prove to be, as is all
of nature's work with which we are thoroughly acquainted, but a
modification and an elaboration of some previously existing process.

Some time ago I was passing the early months of summer by the side of
a lake in northern Pennsylvania. Near my tent, on the edge of the
water, was a wharf from which it was possible to look down into the
shallows about the edge of the lake. In early July the bottom began to
take on a strange appearance. Spots as big as a dinner plate became
evident because they were cleaned of the finer sand or mud which is
common on the bottom. A close examination showed that each of these
circular spots was being occupied and cleaned up by a sunfish. The
pebbles were lifted into the mouth of the fish and driven out again
with force. The water which emerged with the stones seemed to wash
away the dirt, while the pebbles themselves became gradually cleaned
of the green plant life which ordinarily covers them. After the
process was completed each spot was saucer-shaped and free from scum
and mud. Over each of these spots hovered the sunfish which made it,
and round and round the fish swam. The circles thus traversed were so
near each other that every now and then the occupants of two adjoining
nests would meet on the border. The fish which was most nearly on its
own ground would at once attack the other and drive him away. In a few
days the other partner in each family seemed to appear. Now two fishes
swam side by side over each nest, bringing the lower edge of their
bodies comparatively close together. In this position they moved
around over the pebbly bottom. The female was discharging her
multitudinous and very small eggs, so that they dropped to the bottom
of the nest. At the same time the male was expelling what in fish is
known as the milt. In this milt are the sperm cells of the male, each
consisting of a rounded head and a very slender body. These are
attracted by the eggs. Pushing up against them, the head of a sperm
cell, consistin