mark the original lines of their spread, preserved
from generation to generation through the annual lead of the older
birds. If we recollect the Ice-Age which drove the vast majority of the
birds south at the end of the Tertiary, and imagine them later following
the northward retreat of the ice, from their narrowed and overcrowded
southern territory, we may not be far from the secret of the annual
migration.

A more important controversy is conducted in regard to the gorgeous
plumage and other decorations and weapons of the male birds. Darwin, as
is known, advanced a theory of "sexual selection" to explain these.
The male peacock, to take a concrete instance, would have developed its
beautiful tail because, through tens of thousands of generations, the
female selected the more finely tailed male among the various suitors.
Dr. Wallace and other authorities always disputed this aesthetic
sentiment and choice on the part of the female. The general opinion
today is that Darwin's theory could not be sustained in the range and
precise sense he gave to it. Some kind of display by the male in the
breeding season would be an advantage, but to suppose that the females
of any species of birds or mammals had the definite and uniform taste
necessary for the creation of male characters by sexual selection is
more than difficult. They seem to be connected in origin rather with the
higher vitality of the male, but the lines on which they were selected
are not yet understood.

This general sketch of the enrichment of the earth with flowering
plants, insects, and birds in the Tertiary Era is all that the limits of
the present work permit us to give. It is an age of exuberant life
and abundant food; the teeming populations overflow their primitive
boundaries, and, in adapting themselves to every form of diet, every
phase of environment, and every device of capture or escape, the
spreading organisms are moulded into tens of thousands of species. We
shall see this more clearly in the evolution of the mammals. What we
chiefly learn from the present chapter is the vital interconnection of
the various parts of nature. Geological changes favour the spread of
a certain type of vegetation. Insects are attracted to its nutritious
seed-organs, and an age of this form of parasitism leads to a signal
modification of the jaws of the insects themselves and to the lavish
variety and brilliance of the flowers. Birds are attracted to the
nutritious mat