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.
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