it is necessary for us first of all to fix in our minds the idea of
causality. According to the view of all modern science, everything has a
cause. Nothing is uncaused. One event is the result of other previous
events, and is in turn the cause of other events that follow. Yesterday
flowed into to-day, and to-day flows into to-morrow. The world as it
exists to-day is the result of the world as it existed yesterday. This
is true not only of the inorganic world--the world of physics and
chemistry--but it is true of living things as well. The animals and
plants that exist to-day are the descendants of others that lived
before. There is probably an unbroken line of descent from the first
life that existed on the earth to the living forms of to-day.

Not only does the law of causality hold true in the case of our bodies,
but of our minds as well. Our minds have doubtless developed from
simpler minds just as our bodies have developed from simpler bodies.
That different grades and types of minds are to be found among the
various classes of animals now upon the earth, no one can doubt, for the
different forms certainly show different degrees of mentality.
According to the evidence of those scientists who have studied the
remains of animals found in the earth's crust, there is a gradual
development of animal forms shown in successive epochs. In the very
oldest parts of the earth's crust, the remains of animal life found are
very simple. In later formations, the remains show an animal life more
complex. The highest forms of animals, the mammals, are found only in
the more recent formations. The remains of man are found only in the
latest formations.

Putting these two facts together--(1) that the higher types of mind are
found to-day only in the higher types of animals, and (2) that a gradual
development of animal forms is shown by the remains in the earth's