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Updated: May 24, 2025
Here the constituent cells merge their individuality in the common action. We have the first definite many-celled body. It is the type to which a moving close colony of one-celled microbes would soon come. The round surface is well adapted for rolling or spinning along in the water, and, as each little cell earns its own living, it must be at the surface, in contact with the water.
Since this nuclear substance exists in the new cells, and since these generations go on indefinitely, the cells are in a sense "immortal" or deathless. In a one-celled individual, there is no distinction between germinal and bodily functions. But the germ-cells or germplasm continue to be immortal or deathless in the same sense as in the simplest organisms.
It cannot be doubted by any student of nature that the microbe developed so gradually that it is as impossible to fix a precise term for the beginning of life as it is to say when the night ends and the day begins. In the course of time little one-celled living units appeared in the waters of the earth, whether in the shallow shore waters or on the surface of the deep is a matter of conjecture.
The life of this example, representing the whole series of one-celled organisms, is almost infinitely simpler than that of a member of a human community, yet it reveals the beginnings of certain characteristics of the latter.
The lower we descend in the animal scale the more simple are the mechanisms by which results are attained. The one-celled animal may be said to breathe with its whole body, while the man employs a large number of muscles, not to speak, at present, of other arrangements.
To the motionless organism it is no advantage; to the pursuing and pursued organism it is an immense advantage, and is one of the chief qualities for natural selection to foster. For the moment, however, we must glance at the operation of this and other natural principles in the evolution of the one-celled animals and plants, which we take to represent the primitive population of the earth.
The plant world, we saw, starts, like the animal world, with a great kingdom of one-celled microscopic representatives, and the same principles of development, to a great extent, shape it into a large variety of forms. Armour-plating has a widespread influence among them.
Does the occurrence of blastulæ and gastrulæ and one-celled beginnings mean that the higher animals composed of numerous and much differentiated cells have evolved in company from two-layered saccular ancestors which were themselves the descendants of spherical colonies of like cells, and ultimately of one-celled animals?
If plants and animals all developed from a one-celled animal, such as the amoeba, why did not the amoeba develop? Or, if some developed, why not all? Certainly there would not remain a great multitude of species in the microscopic world. Of many species small and large, we have many fossils preserved but no transitional forms.
But we now know that embryology is even more direct in its proof that organic transformation is natural and real; while at the same time there is a completeness in the full series of developmental stages connecting the one-celled egg with the adult creature that must be forever lacking in the case of the fossil sequence of species.
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