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Updated: May 24, 2025
And there are other worlds whose life-cycle has been run; where intelligent life has ceased: where world-disintegration has set in. For this is in accordance with the universal law of Growth and Decay a law that exempts neither the one-celled amoeba, nor the complex Solar system whirling yonder in Infinite space.
With this brief indication of the mechanical principles which have directed the evolution of two of the most remarkable groups of the one-celled animals we must be content, or the dimensions of this volume will not enable us even to reach the higher and more interesting types. We must advance at once to the larger animals, whose bodies are composed of myriads of cells.
A one-celled animal can move about, can recognize the proximity of food, can engulf its food and digest it, can build up its own substance out of the digested food, can absorb oxygen, can use this oxygen in the burning of its own substance to produce its own activities, can act in response to sensation gained from outside, can throw off its waste matter produced by its own activities, and can grow.
The only statement as to our age that the facts would strictly justify us in making must partake of the vagueness of Mr. A. Ward's famous confession that he was "between twenty-three summers." As we individually climb our own family-tree, from the first, one-celled droplet of animal jelly up, none of our organs is older than we are, but a number of them are younger. The appendix is one of these.
In other chapters, particularly the one dealing with the house-fly and typhoid, we shall see how it is that insects are often important factors in spreading some of the most dreaded of the bacterial diseases. The Protozoa, or one-celled animals, belonged to an unknown world before the invention of the microscope.
But we hesitate to say how many of these powers or characteristics of free and independent personality can be assigned to the unicellular organisms, such as the amoeba or the corpuscles of our blood. These one-celled creatures are also alive, are just as truly alive as are those composed of many cells.
We have the one-celled amoeba, the microscopic animals, and the lowest forms of animal life. If the great law of progress and advancement to higher forms has prevailed for so many million years, there should be none but the highest species. All should have reached the status of human beings and there should be none of the lower forms of life which are so abundant.
The lower one-celled organisms in nature increase by division, just as do the individual cells of a more highly organized, many-celled order of living beings. And in all cases, though death or destruction of the cells is synonymous with the death or destruction of the living organism, the latter in most cases already has recreated itself by reproduction.
Stamens many, springing from the side of the tube or throat of the calyx, sometimes joined to the petals, generally equal in length; anthers small and oblong. Ovary smooth, or covered with scales and spines, or woolly, one-celled; style simple, filiform or cylindrical, with a stigma of two or more spreading rays, upon which are small papillae.
Here the assemblage is made up of one-celled individuals essentially similar to one another in structure and in physiological activities; in the latter respect each one of them is like Amoeba as well. They may remain together for a longer or shorter period, or during their whole existence until the time of reproduction.
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