CREATION BY EVOLUTION
The single living cells from which higher life started were for a long time independent creatures, capable of assimilation, growth, and subdivision. After a time, when the daughter cells adhered together, a more or less spherical colonial type appeared, finally attaining regional subdivision and division of labor. Then one side of the sphere grew faster and a pushed-in ball appeared. Hereafter the inner layer served for the digestion of the food and became the primitive gut, while the outer layer not only held the bag together but developed sensory and contractile powers, as in the lower jellyfishes. Meanwhile the puckered skin around the mouth grew out into feelers and stinging tentacles. All this looks simple, but the organization of each individual cell was an affair of unimaginable complexity.
Certain jellyfishes began to give up their free-swimming habits and to squirm or crawl on the muddy bottom. Presently the diffuse “nerve net” throughout the body began to be drawn together into definite tracts, the squirming movements finally became more prominent in one direction, and locomotion in a head-and-tail direction was already begun.
The Origin of the Vertebrates
The vertebrate animals originated millions of years later, and there is as yet no general agreement as to what group of invertebrates gave rise to the vertebrates. Professor E. B. Wilson teaches that the vertebrates (or chordates) belong to that great branch of the animal kingdom in which the mesoderm, or middle layer of the three primary cell-layers, arises from outpockets from the primitive gut, as it does also in the echinoderms (the starfish group) and that all the articulated animals, such as arthropods (crustaceans, insects, and arachnids), annelids (worms), mollusks and other groups belong to a series in which the mesoderm buds off from a
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