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Updated: May 6, 2025
Without leaving the group of one-celled animals typified by Amoeba, we find colonies of the most elementary biological nature, where other natural obligations are added to the two of greatest importance. Some species of the bell-animalcule, Vorticella, provide characteristic examples of these primitive compound protozoa.
It is all very fine to recognise in the ascending series of protozoa, cœlenterata, echinoderms, worms, mollusks, fishes, amphibia, reptiles, the stages of progress toward birds and finally to mammals and man. But whence comes the idea of bird or pigeon?
And, preliminary to the meal, as the prehistoric granite strata heralded the protozoa, the bread of Gaul, compounded after the formula of the recipe for the eternal hills, was there set forth to the hand and tooth of a long-suffering city, while the gods lay beside their nectar and home-made biscuits and smiled, and the dentists leaped for joy in their gold-leafy dens.
There are not very many species of these living gastraeads; but their morphological and phylogenetic interest is so great, and their intermediate position between the Protozoa and Metazoa so instructive, that I proposed long ago to make a special class of them. But we might also regard these three orders as so many independent classes in a primitive gastraead stem.
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.
The protozoa, consisting of one single cell, are supposed to have formed, by assemblage, aggregates which, relating themselves together in their turn, have given rise to aggregates of aggregates; so organisms more and more complicated, and also more and more differentiated, are born of the association of organisms barely differentiated and elementary.
With more or less modification in detail, the embryo has been observed to pass through these successive evolutional stages in sundry Sponges, Coelenterates, Worms, Echinoderms, Tunicates, Arthropods, Mollusks, and Vertebrates; and there are valid reasons for the belief that all animals of higher organisation than the Protozoa, agree in the general character of the early stages of their individual evolution.
A few kinds of disease germs belong to the animal kingdom, though all germs are so tiny that you would have to have a very powerful microscope to tell the difference between the animal germs and the bacilli, or little plants. Most of these animal germs are called protozoa and cause diseases found in, or near, the tropics, like malaria and the terrible "sleeping sickness" of Africa.
The successive individuals thus developed continue attached; give origin to other such individuals which also continue attached; and so there results a compound animal. As in the Hydra itself we find an aggregation of units which, considered separately, are akin to the lowest Protozoa; so here, in a Zoophyte, we find an aggregation of such aggregations.
Can we fail to see the significance of the order in which life has appeared upon the globe the ascending series from the simple to the more and more complex? Can we doubt that each series is the outcome of the one below it that there is a logical sequence from the protozoa up through the invertebrates, the vertebrates, to man? Is it not like all that we know of the method of nature?
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