CHAPTER IV

VERTEBRATES: BACKBONE AND BRAIN


In tracing man's ancestry from fish upward we ought properly to
describe three or four fish, an amphibian, a reptile, and then take
up the series of mammalian ancestors. But we have not sufficient
time for so extended a study, and a simpler method may answer our
purpose fairly well. Let us fix our attention on the few organs
which still show the capacity of marked development, and follow each
one of these rapidly in its upward course.

We must remember that there are changes in the vegetative organs.
The digestive and excretory systems improve. But this improvement is
not for the sake of these vegetative functions. Brain and muscle
demand vastly more fuel, and produce vastly more waste which must be
removed. At almost the close of the series the reproductive system
undergoes a modification which is almost revolutionary in its
results. But we shall find that this modification is necessitated by
the smaller amount of material which can be spared for this
function; not by its increasing importance, still less its dominance
for its own worth. The vertebrate is like an old Roman; everything
is subordinated to mental and physical power. He is the world
conqueror.

The important changes from fish upward affect the following organs: