IT was the great French biologist, Lamarck, who died in 1829, who first proposed the term Biology for the Science of Living Things.
As living things are either Animals or Mints, biology must perforce concern itself with the study of these – and this not in any limited sense, but from the widest possible standpoint. The science must deal not only with the complexities and inter-relations of plants, and with the complexities and interrelations of animals, but, further, with the inter-relations of animals and plants – not only, too, with their dependence one upon another, but with the distinctions between these two great classes of living things.
In the higher forms of life there are certain broad distinctions between Plants and Animals. Among lowly organisms it is sometimes difficult to draw the dividing line. Some, on the border of two kingdoms, as it were, are commonly referred to as plant-animals. This is the case with Euglena, Volvox, and Hydra viridis.
By the uninitiated, movement, that is, progression, would probably be cited as a fundamental quality, marking off an animal from a plant. In the most highly organised forms this distinction holds good. It breaks down when the simplest forms are considered. Chlamydomonas, the plant, moves actively. Hydra, the animal, is stationary for the greater part of its life.
The elaborate digestive, nervous, and respiratory systems of the higher animals have no counterpart in the most highly Structure developed plants. This cannot, however, be re- . garded as a fundamental distinction, for a lowly animal, like a lowly plant, Amceba equally with Chlamydomonas, carries on all physiological activities within the compass of a single cell.
The power of building up organic food is essentially a characteristic of plants. But as it depends upon the presence of chlorophyll, the Mushroom, beyond doubt a plant, is no more capable of working up its own organic food compounds than is the Fish, the Frog, or the Rabbit. The fungi are in exactly the same state of dependence as are animals.
Plants take in their food constituents in fluid form, either as liquid or gas. This is the typical holophytic method of. Animals, on the other hand, holo- zoically, engulfing or swallowing solid particles of food. This does seem a more fundamental distinction than any of the foregoing. But even this is no safe argument, for it is a debatable point whether Euglena really lashes solid food into its gullet, or no. And quite certainly Euglena, claimed by the zoologist and ceded by the botanist, does build up a form of starch, paramylum, by virtue of the chlorophyll granules that are an intimate part of its structure.
Possibly the one unimpeachable distinction between plant and animal is the nature of the waste material excreted in the two cases. Urea is the typical secretion of the animal body. Even in the lowly Amceba it is claimed that uric acid is discharged from the contractile vacuole . No such claim has been made with regard to the similar contractile vacuoles of Chlamydomonas.
It is because of the common ancestry of plants and animals that there is this impossibility of drawing a hard-and-fast line between them. The more primitive types closely resemble the common ancestor and, naturally, each other. In the divergent types individual characteristics are pronounced.
Possibly, the whole question ofis, in the last analysis, responsible for some of the divergences. This is certainly so as far as movement is concerned. Plants have no need to move. The air shifts and fresh supplies of carbon dioxide are brought within their reach. Decay, followed by bacterial action, renews the food content of the soil. Animals, on the other hand, very quickly exhaust the supply of food in one particular area. A tethered goat must be moved frequently. Cows, proverbially, need new pastures,