he eons during which this evolution was going on and which
were its immediate causes.

When the biologist thinks of the evolution of animals and plants, a
different picture presents itself. He thinks of series of animals that have
lived in the past, whose bones (fig. 1) and shells have been preserved in
the rocks. He thinks of these animals as having in the past given birth,
through an unbroken succession of individuals, to the living inhabitants of
the earth today. He thinks that the old, simpler types of the past have in
part changed over into the more complex forms of today.

He is thinking as the historian thinks, but he sometimes gets confused and
thinks that he is explaining evolution when he is only describing it.

[Illustration: FIG. 1. A series of skulls and feet. Eohippus, Mesohippus,
Meryhippus, Hipparion and Equus. (American Museum of Natural History. After
Matthews.)]

A third kind of evolution is one for which man himself is responsible, in
the sense that he has brought it about, often with a definite end in view.

His mind has worked slowly from stage to stage. We can often trace the
history of the stages through which his psychic processes have passed. The
evolution of the steam-boat, the steam engine, paintings, clothing,
instruments of agriculture, of manufacture, or of warfare (fig. 2)
illustrates the history of human progress. There is an obvious and striking
similarity between the evolution of man's inventions and the evolution of
the shells of molluscs and of the bones of mammals, yet in neither case
does a knowledge of the order in which these things arose explain them. If
we appeal to the psychologist he will probably tell us that human
inventions are either the result of happy accidents, that have led to an
unforeseen, but discovered use; or else the use of the invention was
foreseen. It is to the latter process more especially that the idea of
_purpose_ is applied. When we come to review the four great lines of
evolutionary thought we shall see that this human idea of purpose recurs in
many forms, suggesting that man has often tried to explain how organic
evolution has taken place by an appeal to the method which he believes he
makes use of himself in the inorganic world.

[Illustration: FIG. 2. Evolution of pole arms. (Metropolitan Museum. After
Dean.)]

What has the evolution of the stars, of the horse and of human inventions
in common? Only this, that in each case from a simple beginni