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We may imagine that the age of microbes was succeeded by an age of these many-celled larger bodies, and the struggle for life entered upon a new phase. The great principle we have already recognised came into play once more. Large numbers of the many-celled bodies shrank from the field of battle, and adopted the method of the plant.
In most cases, the cells are connected in chains, and we begin to see the vague outline of the larger plant. When we had reached this stage in the development of animal life, we found great difficulty in imagining how the chief lines of the higher Invertebrates took their rise from the Archaean chaos of early many-celled forms.
Because of this care, day by day the little body inside each shell grew from the wonderful single cell it started life with, to a many-celled creature, all fitted out with lungs and a heart and rich warm blood, and very slender legs, and very dear heads with very bright eyes, and all the other parts it takes to make a bird.
In other words, the starting point in the development of the frog is a single biological unit; this divides and its products redivide to constitute the many-celled blastula and the double-walled gastrula.
It exhibits, in a word, the property of irritability that is, simply the power of receiving and reacting to stimuli; and being only a single cell this property is held in common by all of its parts. We come next to a simple many-celled animal like the polyp Hydra, or a jellyfish.
As this is the course of embryonic development, and as it is so well retained in the lowest groups of the many-celled animals, we take it to be the next stage. The reason for it will become clear on reflection. Division of labour naturally takes place in a colony, and in that way certain cells in the primitive body were confined to the work of digestion.
Here the constituent cells merge their individuality in the common action. We have the first definite many-celled body. It is the type to which a moving close colony of one-celled microbes would soon come. The round surface is well adapted for rolling or spinning along in the water, and, as each little cell earns its own living, it must be at the surface, in contact with the water.
The Sponges are a side-issue, or cul de sac, from the Protozoic world, and do not lead on to the higher. Each one-celled unit remains an animal; it is a colony of unicellulars, not a many-celled body. We may admire it as an instructive approach toward the formation of a many-celled body, but we must look elsewhere for the true upward advance.
Yet all the many-celled organisms that we are so accustomed to regard as individuals are really communities, demonstrating the existence and partial antithesis of the great laws of egoism and altruism, which are traceable even down to Amoeba and its like.
For a time, under favorable circumstances, volvox reproduces very much like magosphæra, and each cell can give rise to a new, many-celled individual. But after a time, especially under unfavorable circumstances, a new mode of reproduction appears. Certain cells withdraw from the outer layer into the interior of the colony.
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