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Those who accept evolution insist on unbroken physical continuity between the earliest known life and ourselves, so that we both are and are not personally identical with the unicellular organism from which we have descended in the course of many millions of years, exactly in the same way as an octogenarian both is and is not personally identical with the microscopic impregnate ovum from which he grew up.
Nevertheless, consciousness and unconsciousness mark the directions in which the two kingdoms have developed, in this sense, that to find the best specimens of consciousness in the animal we must ascend to the highest representatives of the series, whereas, to find probable cases of vegetable consciousness, we must descend as low as possible in the scale of plants down to the zoospores of the algae, for instance, and, more generally, to those unicellular organisms which may be said to hesitate between the vegetable form and animality.
Hence the comparative study and classification of modern races establishes a scale of social grades which corresponds with the order of their historic succession, just as in a larger way the complete series of comparative anatomy from Amoeba to man displays the order of evolution from unicellular beginnings to the present culminating types.
If, in a froth of the same kind, the air is extracted from an alveolus, a cone of attraction is seen to form, like those about the centrosomes which result in the division of the nucleus. Even the external motions of a unicellular organism of an amoeba, at any rate are sometimes explained mechanically.
To explain this Weismann went back to the unicellular protozoa. These animals are undoubtedly influenced by environment and vary under its stimuli. Here the variations were stamped upon the germ-plasm, and the commingling of these variously stamped germ-plasms has resulted in all the variations of higher animals. Of late Weismann has modified and greatly improved this portion of his theory.
Like these unicellular Protozoa, the colourless blood-cells creep slowly about, their unshapely plasma-body constantly changing its form, and stretching out finger-like processes first in one direction, then another. Like the Amoebae, they take particles into their cell-body.
These he illustrates mainly by examples from his own special domain and personal observation. The simplest unicellular forms of life are to be thought of as at the beginning of evolution; and, since mechanical causes cannot explain their ascent, it must be assumed that they have an inherent “phylogenetic potential of development,” which, working epigenetically, results in ascending evolution.
The reproduction of unicellular organisms consists in just this the living being divides into two halves, of which each is a complete individual. True, in the more complex animals, nature localizes in the almost independent sexual cells the power of producing the whole anew.
It may therefore be useful to consider one or two intermediate forms and the parallel embryonic stages of higher animals, and to see how the higher many-celled animal originates from the unicellular stage. The amoeba is an illustration of a great kingdom of similar, practically unicellular forms, which have played no unimportant part in the geological history of the globe. These are the protozoa.
There is yet another of zoophytes, or polyps, and another of sponges, and, finally, we have a sub-kingdom of minute creatures, or animalculæ, of very varied forms, which may make up the sub-kingdom of Protozoa, consisting of animals which are mostly unicellular.
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