doubt performed certain brain functions. But the principle of
centralization was at work, and when in later time we reach the
higher mammalian forms, we find these outlying nervous masses called
in, so to speak, and concentrated in the head.

Nature has tried the big, the gigantic, over and over, and then
abandoned it. In Carboniferous times there was a gigantic
dragon-fly, measuring more than two feet in the expanse of wings.
Still earlier, there were gigantic mollusks and sea scorpions, a
cephalopod larger than a man; then gigantic fishes and amphibians
and reptiles, followed by enormous mammals. But the geologic record
shows that these huge forms did not continue. The mollusks that last
unchanged through millions of years are the clam and the oyster of
our day. The huge mosses and tree-ferns are gone, and only their
humbler types remain. Among men giants are short-lived.

On the other hand, the steady increase in size of certain other
species of animals during the later geologic ages is a curious and
interesting fact. The first progenitors of the elephant that have
been found show a small animal that steadily grew through the ages
till the animal as we now find it is reached. Among the
invertebrates this same progressive increase in size has been noted,
a small shell in the Devonian becoming enormous in the Triassic.
Certain species of sharks of medium size in the lower Eocene
continue to increase till they attain the astounding dimensions in
the Miocene and Pliocene of over one hundred feet long. A certain
fish appearing in the Devonian as a small fish of seven centimetres
in length, becomes in the Carboniferous era a creature twenty-seven
centimetres in length. Among the mammals of Tertiary times this same
law of steady increase in size has been operative, as seen in the