motion, and to dart after a fly with the swiftness of an arrow--all this
wondrous mechanism, all this beauteous structure, all this perfection of
function, all this adaptation to environment, have evolved from a few
microscopic cells in three short weeks.

Biology is the science that observes all this, and enunciates the law
that the life history of this animal cell, _i.e._, its history from a
simple unicellular state in the egg, to its complex multicellular state
in the matured chick, represents the history of the race to which the
chick belongs. If we could trace that chicken back through all its
ancestry, we would discover at different periods in the history of life
upon the globe (about 100 million years, according to Haeckel) exactly
the stages of development we found in the life history of the chick, and
arrive at last at a primordial cell.

What is true of the chick is true of all life. This is the law of
evolution. It is true of all plant and animal life; it is true of man as
an individual; it is true of his mind as well as of his body; it is true
of society as an aggregation of individuals. As men have evolved from a
lower to a higher, a simple to a complex state, so they are still
evolving and rising "on stepping stones of their dead selves to higher
things."

Natural selection, or the survival of the fittest, is one of the
processes by which evolution takes place. According to this law, only
the fittest survive in the struggle for life. Darwin was led to this
discovery on reading Malthus's thesis regarding the disproportion
between the rates of increase in population and food, and the consequent
struggle for existence.