At the lower end of the animal scale are organisms which consist of one cell and nothing more. Amoeba, to which we must refer again and again, is an example of this group which possesses an overwhelming importance to the comparative student because the origins of all the characteristics of animals higher in the scale are to be found within it. Amoeba itself is a naked mass of protoplasm, about 1/100 of an inch in diameter, enclosing a nucleus.  Its form is not constant during activity, for fingerlike processes called pseudopodia are pushed out tentatively in many directions to be followed as circumstances direct by the materials of the whole cell body.  Other protozoa differ in possessing constant forms, or in having constant vibratile processes, or shells of some kind, while in still other cases like individuals combine to make colonies which are more or less definite and permanent.  Here at the very foot of the organic scale are found animals which seem to be entirely different from those above.  Upon examination they, like Hydra, prove to be the same as regards the number and kind of functions they perform, but in structural regards their evolutionary relation to all higher animals is indicated solely by the fact that they are cells composed of protoplasm.  Nevertheless the principle which states that resemblance means consanguinity still holds true, for cellular constitution is a unique possession of things of the living world,—­something which demonstrates the common origin of all living things just as truly as the “cat-ness” of our first series of examples reveals for a smaller group the significance of likeness and the nature of the basic law of comparative anatomy.

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Employing a figure of speech, we have climbed down the animal tree from the higher regions where the mammals belong.  Having reached the very foot of the trunk we are in a position to review and summarize the evidences which we have discovered all about us as we have descended.  The various examples we have mentioned and the groups to which they belong clearly occupy different places in the scale which begins with the protozoa and extends upward to the most complicated and differentiated animals. Hydra takes its place above the protozoa for obvious structural reasons; worms belong to a still higher zone, surpassed by the more complex jointed animals like crustacea and insects.  Far above these are the vertebrates, among which we have already demonstrated the occurrence of different grades of organization, from the fish up to the higher amphibia and reptiles, and beyond in two directions to the diverging birds and mammals.  The basic characteristics of every group in a high position may be traced back to some one or another of the divisions at a lower level, so that the general sequence of the structural levels from low to high becomes intelligible as the order of their evolution.