live that function still is, is grossly shown by the swelling of
the gland with sexual excitement, menstruation and pregnancy.

Relative to the body weight it is largest in the mammalia, and
smallest in the fishes. It therefore grows larger as the vertebrate
ascends in the scale. It has, in fact, developed in direct proportion
to and side by side with the fundamental, differentiating vertebrate
characteristics. Of these, the possession of a dry hairy skin instead
of a moist or mucus bearing, chitinous skin, the ownership of
an internal bony skeleton and a large skull, and a complicated
development of brain, are the diagnostic signs. Thyroid internal
secretion has a very definite controlling relation to all of them: to
skin, its hairiness, moisture and amount of mucus, to the growth and
size of the bones, especially the bones of the extremities and the
skull, and to intelligence and the complexity of the convolutions of
the brain. Injury to the thyroid, especially in growing animals, is
followed by profound retrogression or arrest of development in skin,
skeleton and brain.

In the fishes and the cyclostomes the thyroid is represented only by
some small scrubby patches, little larger than the heads of pins,
scattered along the aorta, the great blood vessels from the heart, and
out a little way along each gill. It becomes larger and more compact
among the amphibians and reptiles, but still remains quite small.
Large and prominent among the birds and mammalia, it is largest and
most prominent among the primates and man. It is hence permissible to
think of the thyroid as a dictator of evolution, to crown it as the
vertebrate gland par excellence, and to call the typical vertebrate
brand marks secondary _thyroid_ characteristics in precisely the
sense of Darwin classing the horns of cattle as secondary _sexual_
characteristics.

In such enthusiasm for the thyroid as a determinant of evolution, its
pillar of cloud by day and column of fire by night, one should not
forget the other glands of internal secretion. In them all, we may
suppose, Life, tired of inventing merely prehensile, destructive and
reproductive organs, hit upon the happy thought of contrivances which
are in essence chemical factories to speed up the rate of variation
and so of a higher evolution.

CREATOR OF THE LAND ANIMAL

According to this conception the thyroid played a fundamental part in
the change of sea creatures into land animals. Experimentally