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Updated: May 23, 2025
If we take an infusorian sufficiently large, such as the Stentor, and cut it into two halves each containing a part of the nucleus, each of the two halves will generate an independent Stentor; but if we divide it incompletely, so that a protoplasmic communication is left between the two halves, we shall see them execute, each from its side, corresponding movements: so that in this case it is enough that a thread should be maintained or cut in order that life should affect the social or the individual form.
As for the temporary prolongations or pseudopodia which the amoeba seems to make, they would be not so much given out by it as attracted from it by a kind of inhalation or suction of the surrounding medium. In the same way we may perhaps come to explain the more complex movements which the Infusorian makes with its vibratory cilia, which, moreover, are probably only fixed pseudopodia.
And it may be asked what advantage, as far as we can see, would it be to an infusorian animalcule to an intestinal worm or even to an earth-worm, to be highly organised. If it were no advantage, these forms would be left, by natural selection, unimproved or but little improved, and might remain for indefinite ages in their present lowly condition.
Even to-day, perhaps, a chlorophyl-bearing Infusorian such as the Euglena may symbolize this primordial tendency of life, though in a mean form, incapable of evolving. Is the divergent development of the two kingdoms related to what one may call the oblivion of each kingdom as regards one of the two halves of the programme?
In the eighteenth century naturalists were very keen on the Infusorian Amoebas, and were much struck by the way in which the members of this old family behaved and developed.
If, on the other hand, an internal activity is appealed to, then it must be something quite different from what we usually call an effort, for never has an effort been known to produce the slightest complication of an organ, and yet an enormous number of complications, all admirably coördinated, have been necessary to pass from the pigment-spot of the Infusorian to the eye of the vertebrate.
Now, if I pass from the top to the bottom of the scale of living beings, from one of the most to one of the least differentiated, from the multicellular organism of man to the unicellular organism of the Infusorian, I find, even in this simple cell, the same process of growing old.
When we think of the enormous number of variations, all in the same direction, that we must suppose to be accumulated before the passage from the pigment-spot of the Infusorian to the eye of the mollusc and of the vertebrate is possible, we do not see how heredity, as we observe it, could ever have determined this piling-up of differences, even supposing that individual efforts could have produced each of them singly.
The Infusorian is exhausted at the end of a certain number of divisions, and though it may be possible, by modifying the environment, to put off the moment when a rejuvenation by conjugation becomes necessary, this cannot be indefinitely postponed.
It aims at discovering the mathematical world-formulæ, if not indeed one great general formula which embraces, defines unequivocally, and rationalises all the processes of and in infinity, from the movements of Sirius to those of the cilia of the infusorian in the drop of water, and which not only crowds “heaven” out of the world, but strips away from things the fringe of the mysterious and incommensurable which seemed to surround them.
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