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Updated: May 20, 2025
Those who examine the main facts of animal and vegetable organization 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.
You follow the progress of the force which is stronger than hunger and cold and swifter and more final than death, from its potential state in the unicellular stage where life goes on by division, up through the multifarious forms of instinctive animal mating, till you reach the love of the sexes in the human world.
The Family rests upon the Great Biological Fact of Sex. While sex does not characterize all animal forms, still it does characterize all except the simplest forms of animal life. These simplest forms multiply or reproduce by fission, but such asexual reproduction is almost entirely confined to the unicellular forms of life.
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.
The absorption of the chyle through the walls of the intestine seemed to be a mechanically intelligible process of osmosis and diffusion. But in reality it proves to be rather a process of selection on the part of the epithelial cells of the intestine, analogous to the selection and rejection exercised elsewhere by unicellular organisms.
That unicellular algae, like the Diatomaceae, living in a medium which may contain only one part in 10,000 by weight of dissolved silica, or even less than that amount, should be able to separate this substance to form their exquisitely ornamented frustules is one of the most striking facts in natural history, whether we regard it in its physiological or its chemical aspects.
If we admit that a unicellular germ of life or a bioplasm, after passing through various stages of evolution, has ultimately manifested in the form of a highly developed human being, then we shall have to admit the potentiality of all the manifested powers in that germ or bioplasm, because the law is "that which exists in the end existed also in the beginning."
In different parts of the vegetable kingdom, and even in different families this progression takes place on different lines. By this means it results in an ever increasing divergency between the several groups. Every step is an advance, and many a step must have been taken to produce flowering plants from the simplest unicellular algae.
The first fact that stands out clearly is that the nucleus is connected most intimately with the process of reproduction and especially with heredity. This has long been believed, but has now been clearly demonstrated by the experiments of cutting into fragments the cell bodies of unicellular animals.
The protozoa include also the flagellata, a great, very poorly defined mass of forms occupying the boundary between the plant and animal kingdoms. They are usually unicellular, and their protoplasm is surrounded by a thin, structureless membrane. This prevents their putting out pseudopodia as organs of motion.
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