the plant has tissues composed of cellulose, or of binary and ternary compounds.
With these may be compared the subjoined parallel considerations:—
1. The Corynidæ, Sertularidae, and many other undoubted animals, are fixed to foreign supports, that is, rooted, just as Laminaria and most sea-weeds are rooted. The common Duckweed is not rooted: is it, therefore, not a plant (?)
The Tœniadæ and Acanthocephala have neither mouth nor stomach. The males of Rotifers are in a similar predicament. Are such organisms plants?
3. Plants exhale oxygen, it is true; but they also, like animals, exhale carbonic acid. The experiments of Saussure are conclusive upon this point.
4. Every plant contains nitrogen in its tissues. According to the analysis of Chevandier, wood yields from 0⋅67 to 1⋅52 of nitrogen. And in an approved Manual of Chemistry we read,
"That certain of the azotised principles of plants, which often abound, and are never altogether absent, have a chemical composition and assemblage of properties which assimilate them in the closest manner, and, it is believed, even identify them, with the azotised principles of the animal body: vegetable albumen, fibrin, and casein, are scarcely to be distinguished from the bodies of the same name extracted from blood and milk."
And in the tests of Ascidians, a deposit of cellulose takes place, precisely after the manner of its formation in the tissues of plants.
So much, then, for Professor Owen's distinctions between the animal and vegetable kingdoms. They prepare us to understand his implied definition of the organisms included in his new kingdom of Protozoa. These "manifest the common organic characters," or, in other words, perform the vital act of nutrition, "but without the distinct super-additions of plants and animals." It follows, therefore, as a necessary inference from the quotations above made, that the anomalous beings in question neither move nor are rooted, but remain in some peculiar physical condition yet to be explained; that they do not receive nutritive matter by a mouth, and, at the same time, differ from organisms which have neither mouth nor stomach; that they neither inhale nor exhale oxygen; and that neither binary, ternary, nor quaternary compounds enter into the composition of their tissues. Such, according to Professor Owen, are the distinctive characteristics of the organic kingdom, Protozoa.
We conclude, however, that a line of demarcation exists between the animal and vegetable kingdoms, and that the Protozoa, rightly so called, have their place on the animal side of the line. The unprejudiced reader of Lieberkühn's careful memoirs can no longer remain in doubt as to the animal nature of the Sponges. And it is for him who disputes the vegetability of the Diatoms and Desmids to set aside the long series of observations inaugurated by the positive discoveries of Thwaites and Ralfs. The difficulty of expressing, by definition, the distinctions between plants and animals rests, be it remembered, more
