The Plant's Place In Nature. Part 197. The Three Kingdoms

Part 197. The three kingdoms. It has long been the general opinion that all natural objects fall readily into three main groups or kingdoms-the mineral, the vegetable, and the animal. Over a century ago the characteristics of each kingdom as understood at the time, were given by Linnaeus in his famous aphorism: "Minerals grow; plants grow and live; animals grow, live, and feel."¹ This threefold division is still recognized as convenient, and the distinctions given are admitted as valid to a considerable extent; but that a mineral grows in essentially the same way as a plant, and that a plant lacks any quality that is found in all animals, would not generally be admitted by the naturalists of to-day. In order to understand modern views regarding the plant's place in Nature we need to consider what is meant by growing, living, and feeling.

¹ Lapides crescunt; vegetabilia crescunt et vivunt; animalia crescunt, vivunt et sentiunt.

By "growth" Linnaeus seems to have meant merely increase in size. Yet is not the enlargement of a seaweed or a fish essentially different from the so-called growth of a salt crystal in concentrated brine? The crystal gets larger simply by additions upon the outside, while the living body increases in size by the incorporation within itself of substances derived from without. Moreover, the crystal as it enlarges remains substantially the same throughout, and all the parts behave alike. In a growing body on the contrary there is a progressive differentiation of parts and functions. Hence we cannot say that a mineral grows in the same sense that an organism grows.

But does not Linnaeus express the differences above insisted upon by saying that organisms are alive? Doubtless he meant to do so; yet what did he mean by life without any trace of feeling? What sort of feeling can a sponge or a jellyfish have that we must deny to a climbing-plant, or to a swimming-plant that moves toward the light? Our only evidence that the animal feels is that it responds by movements to certain stimuli. When we watch plants carefully we find that they also respond to similar stimuli. Thus we are left without any distinction between plants and animals; and since what "feeling" stands for in animals is found in plants as well, it would seem that this same "feeling" might be what best distinguishes living from lifeless bodies, and so underlies the various manifestations of life. According to a view which we must examine more at length it is because of their purposeful activities that animals and plants are called living, and because of their coordinated parts, organic. All other bodies are then appropriately termed lifeless or inorganic. This modern view of Nature implies a revised classification which may be conveniently presented in the following tabular form.