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Updated: May 20, 2025
The origin and development of the blastoderm or germinal disc has nothing to do with recapitulation of the phylogeny. It is not the case that the embryos of higher vertebrates are indistinguishable from one another. Even the egg-cell has a specific character, and is totally different from any unicellular organism at the Protistan level.
We have learned that the human body is a confederation of various groups of living cells; that in the earliest stages of man's evolution, these cells were all of the same general type; that as such they were free-living, free-thinking and intelligent organisms as certainly as were those unicellular organisms which had not become members of any group or association; that through the processes of evolution, heredity and adaptation, there has come about in the course of the ages, a subdivision of labor among the cells of our bodies and a consequent differentiation in kind whereby each has become peculiarly fitted for the performance of its allotted functions; that, nevertheless, these cells of the human body are still free-living, intelligent organisms, of which each is endowed with the inherited, instinctive knowledge of all that is essential to the preservation of its own life and the perpetuation of its species within the living body; that, as a part of the specializing economy of the body, there have been evolved brain and nerve cells performing a twofold service first, constituting the organ of a central governing intelligence with the important business of receiving, classifying, and recording all impressions or messages received through the senses from the outer world, and, second, communicating to the other cells of the body such part of the information so derived as may be appropriate to the functions of each; that finally, as such complex and confederated individuals, each of us possesses a direct, self-conscious knowledge of only a small part of his entire mental equipment; that we have not only a consciousness receiving sense impressions and issuing motor impulses through the cerebro-spinal nervous system, but that we have also a subconsciousness manifesting itself, so far as bodily functions are concerned, in the activity of the vital organs through the sympathetic nerve system; that this subconsciousness is dependent on consciousness for all knowledge of the external world; that, in accordance with the principles of evolution, man as a whole and as a collection of cell organisms, both consciously and unconsciously, is seeking to adapt himself to his external world, his environment; that the human body, both as a whole and as an aggregate of cellular intelligences, is therefore subject in every part and in every function to the influence of the special senses and of the mind of consciousness.
Such is the ultimate structure, and such are the fundamental activities or functions of living organisms, as far as they can here be described, from the lowest animalcule and unicellular plant, up to the most complex organisms and the body of man himself.
The origin of sexual reproduction in animals must have been something as follows: The first method of reproduction was by a simple division of the unicellular organism to form two new individuals. At times, a fusion of two independent individuals occurred. This was known as conjugation, and is seen among Paramecia and some other species to-day.
Probably the Noctiluca is not rivalled in this respect even by miscroscopic unicellular algæ which compose the "red snow." By filtering sea-water containing Noctilucæ its light can be concentrated, and it has been found that a few teaspoonfuls will then yield light enough to enable one to read holding a book at the ordinary distance from the eyes about ten inches.
Hugo von Mohl applied to the fluid contents of the cell the term "protoplasm," and Max Schultze showed that this protoplasm is really identical in all organisms, plants and animals, also that the cell-wall is frequently absent in many animal tissues and in many unicellular forms, indicating that the protoplasm is the really important substance.
The transit, we are told, from the unicellular to the multicellular organism cannot be explained by individualism, but implies a diminution of the competitive, an increase of the social and subordinative tendency.
And on passing from what are roughly classed as unicellular organisms to the lowest of those which consist of aggregated cells, we equally observe the connexion between structural differences and differences of circumstance. In the sponge, permeated throughout by currents of sea-water, the absence of definite organization corresponds with the absence of definite unlikeness of conditions.
But do we not know enough to see that it did not originate in this sudden spectacular way? that it began very slowly, in unicellular germs?
Why should the vertebrate series have risen through the fish, the reptile, the mammal, to man, unless the manward impulse was inherent in the first vertebrate; something that struggled, that pushed on and up from the more simple to the more complex forms? Why did not unicellular life always remain unicellular?
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