Each great
group of animals has had successively its rise, its zenith, its
decadence, and its dotage; each at the period of its highest development
has produced a considerable number of colossal forms; each has been
supplanted in due time by higher groups of totally different structure,
which have killed off their predecessors, not indeed by actual stress of
battle, but by irresistible competition for food and prey. The great
saurians were thus succeeded by the great mammals, just as the great
mammals are themselves in turn being ousted, from the land at least, by
the human species.
Let us look briefly at the succession of big animals in the world, so
far as we can follow it from the mutilated and fragmentary record of the
geological remains.
The very earliest existing fossils would lead us to believe what is
otherwise quite probable, that life on our planet began with very small
forms--that it passed at first through a baby stage. The animals of the
Cambrian period are almost all small mollusks, star-fishes, sponges, and
other simple, primitive types of life. There were as yet no vertebrates
of any sort, not even fishes, far less amphibians, reptiles, birds, or
mammals. The veritable giants of the Cambrian world were the
crustaceans, and especially the trilobites, which, nevertheless, hardly
exceeded in size a good big modern lobster. The biggest trilobite is
some two feet long; and though we cannot by any means say that this was
really the largest form of animal life then existing, owing to the
extremely broken nature of the geological record, we have at least no
evidence that anything bigger as yet moved upon the face of the waters.
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