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Updated: May 20, 2025
Now, for some of the ancestral stages of man's development a very high degree of probability can be claimed. One of man's earliest ancestors was almost certainly a unicellular animal. A little later he very probably passed through a gastræa stage. He traversed fish, amphibian, and reptilian grades.
Unicellular forms can come only from preexisting cells of the same kind; and even the individual cells of a multicellular organism, when once differentiated, reproduce only other cells after their own kind.
Can we conceive of an indestructible thing be it unicellular or multicellular, alive or robotic, composed of independent individuals or acting in perfect, centrally-dictated unison? Can anything be, in principle, eternal? This question is not as outlandish as it sounds. By fighting disease and trying to postpone death, for instance, we aspire to immortality and imperishability.
Those who examine the main facts of animal and vegetable organisation without bias will, no doubt, ere long conclude that all animals and vegetables are derived ultimately from unicellular organisms, but they will not less readily perceive that the evolution of species without the concomitance and direction of mind and effort is as inconceivable as is the independent creation of every individual species.
Herbert Spencer would probably answer that, as soon as simple unicellular organisms came by growth or division to be compounded of several cells, or became attached to any supporting surface, his law "that homologous units of any order become differentiated in proportion as their relations to incident forces become different" would come into action.
Others permanently formed of some multiple of four; the highest forms made up of compact, numerous, more or less closely joined cells. Reproduction: by cell division, by the conversion of the cell contents into zoospores, and by resting spores, formed sometimes after conjugation; in other cases, probably, by fecundation by spermatozoids. All the unicellular algae are included under this head.
The pluricellular organism is nothing more nor less than a later development, a confederated association of unicellular organisms. Mark the development of such an association. Originally each separate cell performed all the functions of a separate life. The bonds that united it to its fellows were of the most transient character.
The first cell-communities to be formed, which laid the early foundation of the higher multicellular body, must have consisted of homogeneous and simple amoeboid cells. The oldest Amoebae lived isolated lives, and even the amoeboid cells that were formed by the segmentation of these unicellular organisms must have continued to live independently for a long time.
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.
There is a circulatory system, a respiratory system, etc.; but we must once more point out that such systems are made up of cells, so that every function of the highest animal may be finally reduced to what takes place in the unicellular animal.
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