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Updated: May 20, 2025
Large unicellular organisms like the Amoebae were found creeping about inside the body of the sponge, and were thought to be parasites. It was afterwards discovered that they were really the ova of the sponge from which the embryos were developed.
Among unicellular animals are some which are large enough for direct manipulation, and it is found that if these cells are cut into pieces the different pieces will behave very differently in accordance with whether or not they have within them a piece of the nucleus. All the pieces are capable of carrying on their life activities for a while.
In this fact, and the fact we have already established that the Gastraea has been evolved from the hollow vesicle of the one-layered Blastaea, and this again from the original unicellular stem-form, we have obtained a solid basis for our study of evolution. The second section, that leads from the Gastraea to the Prochordonia, is much more difficult and obscure.
The egg, in the modern physiological sense of the word, did not make its appearance until the descendants of the unicellular Protozoon had developed into multicellular animals, and these had undergone sexual differentiation.
The earliest forms of Life, the unicellular "Beings," whether animal or vegetable for both divisions, if they can be said to be divided, have the same protoplasmic cell as basis of life were, and are still, immortal except for accidents; they are not subject to natural death as we know it; they multiply by fission and not by "budding."
They point out that a man is made up of a great multitude of cells, each equivalent to a unicellular organism. Not one of those cells is he, nor is he simply just the addition of all of them. He is more than all of them. You can take away these and these and these, and he still remains.
And when this single cell proceeds through successive embryonic stages to develop into an adult individual it naturally, through force of hereditary habit, so to speak, treads the same path which its ancestors followed from the unicellular condition to their present point of development. Thus higher forms should be expected to show traces of their early ancestry in their embryonic life.
Those long ages of unicellular life in the old seas, how immense they appear to have been; then how the age of invertebrates dragged on, millions upon millions of years; then the age of fishes; the Palaeozoic age, how vast put by Haeckel at thirty-four millions of years, adding rock strata forty-one thousand feet thick; then the Mesozoic or third period, the age of reptiles, eleven million years, with strata twelve thousand feet thick.
He tells us that large numbers of the lower animals have no nervous system, though they exercise the functions of sensation and will like the higher animals. In the unicellular Protozoa, which do not form germinal layers, there is, of course, neither nervous system nor skin. But in the second division of the animal kingdom also, the Metazoa, there is at first no nervous system.
Many millions of years had to pass before the most advanced vertebrate, man, was evolved, step by step, from his ancient unicellular ancestors.
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