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Updated: May 6, 2025
These "discoveries" have ranged the whole realm of unicellular life, bacilli, bacteria, spirilla, yeasts, moulds, protozoa, yet the overwhelming judgment of broad-minded and reputable experts the world over is the Scotch verdict of "not proven"; and we are more and more coming to turn our attention to the other aspect of the problem, the factors which cause or condition this isolation and assumption of autonomy on the part of the cells.
We might almost claim that if the evolution of man from the single cell be granted, no other order of their occurrence is possible. The protozoa are mostly, though not purely, nutritive and reproductive. These functions are essential to the existence of the species. Naturally in the early protozoan colonies, and in forms like hydra, these functions predominated.
Bivalve and univalve mollusca seem to be rare at the greatest depths; but starfishes, sea urchins and other echinoderms, zoophytes, sponges, and protozoa abound. It is obvious that the Challenger has the privilege of opening a new chapter in the history of the living world. She cannot send down her dredges and her trawls into these virgin depths of the great ocean without bringing up a discovery.
At any rate, the protoplasm of which they are all alike composed is identical wherever found, so far as any scientific tests have yet been able to determine. There are many varieties of single cells known to science which maintain an independent individual existence. Among the unicellular plants are the bacteria, while the unicellular animals are known as the protozoa.
Among the diseases which have been proved to be due to protozoa are malaria, amoebic dysentery, and syphilis; while among the much larger number which are due to bacteria, bacilli, or other vegetable parasites, are cholera, typhoid fever, the plague, pneumonia, diphtheria, tuberculosis, and leprosy.
If I had worked my way from a sponge instead of a lobster, I should have found it associated, by like ties, with a great number of other animals into the sub-kingdom Protozoa; if I had selected a fresh-water polype or a coral, the members of what naturalists term the sub-kingdom Coelenterata, would have grouped themselves around my type; had a snail been chosen, the inhabitants of all univalve and bivalve, land and water, shells, the lamp shells, the squids, and the sea-mat would have gradually linked themselves on to it as members of the same sub-kingdom of Mollusca; and finally, starting from man, I should have been compelled to admit first, the ape, the rat, the horse, the dog, into the same class; and then the bird, the crocodile, the turtle, the frog, and the fish, into the same sub-kingdom of Vertebrata.
It would seem that Cupid would find these ocular vampires too cold game for his calorific shafts, but have we not yet to discover an immune even among the Protozoa? Yes, beautiful Romance descended upon two of this tribe, and love came into their hearts as they crowded about the prostrate form of a man who had been run over by a brewery wagon. William Pry was the first on the spot.
Restricting attention to the three primary examples, and remembering that the criterion of completeness is the ability to discharge satisfactorily all of the eight biological tasks, it is clear that the entire Hydra and the whole ant-community correspond physiologically with Amoeba, although the first-named is structurally a cell-community equivalent to many protozoa, and the insect colony is composed of many such cell-communities as elements.
And these cells, forming a series through all generations, are evidently immortal like the protozoa. Natural death cannot touch them. These are the reproductive cells. The other cells nourish and transport them and carry on the work of excretion and respiration. These latter correspond practically to our whole body. We call them somatic cells.
Man's ancestors are round coenobia or colonies of Protozoa; they consist of a close association of many homogeneous cells, and thus are individuals of the second order. Their body consists merely of a primitive gut, the wall of which is made up of the two primary germinal layers. The unsegmented chorda develops between the dorsal medullary tube and the ventral gut-tube. Head-gut with gill-clefts.
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