so far back as 1851, the existence of true ova was conclusively demonstrated in the genus Tethya.
Those who refuse to admit the Protozoa within the animal kingdom are bound legitimately to solve the question which they have raised as to their true systematic position. Professor Agassiz, as we have seen, adopts the ready expedient of banishing the majority of the group to the vegetable kingdom. Botanists, however, refuse to acknowledge the outcasts thus summarily thrust upon them for protection. Our great anatomist, Professor Owen, has proposed to cut short the difficulty, by establishing for their reception a third primary division of the organic world. By him we are informed that—
"The two divisions of organisms called 'plants' and 'animals' are specialized members of the great natural group of living things; and there are numerous beings, mostly of minute size, and retaining the form of nucleated cells, which manifest the common organic characters, but without the distinctive super-additions of plants and animals. Such organisms are called 'Protozoa,' and include the Sponges or Amorphozoa, the Foraminifera or Rhizopods, the Polycystineæ, the Diatomaceæ, Desmidiæ, Gregarina, and most of the so-called Polygastria of Ehrenberg, or infusorial animalcules of older authors."
The "common organic characters" here alluded to have, in a preceding paragraph been denned as follows:—
"Organisms, or living things, are those which possess such an internal cellular or cellulo-vascular structure as can receive fluid matter from without, alter its nature, and add it to the alterative structure. Such fluid matter is called 'nutritive,' and the actions which make it so are called 'assimilation' and 'intra-susception.' These actions are classed as 'vital,' because, as long as they are continued, the 'organism' is said 'to live.'"
Professor Owen then goes on to distinguish between plants and animals thus.—
"When the organism can also move, when it receives the nutritive matter by a mouth, inhales oxygen, and exhales carbonic acid, and developes tissues, the proximate principles of which are quaternary compounds of carbon, hydrogen, oxygen, and nitrogen, it is called an 'animal.' When the organism is rooted, has neither mouth nor stomach, exhales oxygen, and has tissues composed of 'cellulose' or of binary or ternary compounds, it is called a 'plant.'"
To do justice to these definitions, we shall, without alteration of the author's language, present them in the form of the four following propositions:—
1. The animal can move; the plant is rooted.
2. The animal receives nutritive matter by a mouth; the plant has neither mouth nor stomach.
3. The animal inhales oxygen, and exhales carbonic acid; the plant exhales oxygen.
4. The animal developes tissues, the proximate principles of which are quaternary compounds of carbon, hydrogen, oxygen, and nitrogen;
