Page:Popular Science Monthly Volume 14.djvu/42

32 the three kingdoms—the mineral, the vegetable, and the animal—as three successive steps in an ascending series, each being supposed to have all the properties and characteristics of the groups below it, and something new and entirely different in addition. This conception admits of such brief and definite statement, and when put into words is so clear and simple, that its general acceptance is quite natural; and we need not be surprised that it only gives way very slowly in favor of a view which does not admit of the same definite and simple formulation.

We will now examine the reasons which have led modern thinkers to reject this view, and to hold that, real and actual as the differences between animals and plants are, they are by no means absolute.

The most conspicuous and superficial difference between animals and plants is the one which we have already noticed. Animals have the power of free locomotion and of independent action, which is determined within the animal; these powers are called into action by changes in the external world, and imply the existence of sensation and consciousness. In all the ordinary plants the power of locomotion is lacking, and there are no voluntary actions like those which result from the sensitiveness of the animal. This difference finds its expression in the well-known dictum of Linnæus: "Plantæ vivunt; animalia vivunt et sentiunt;" and upon it is based the classification of the functions and organs of the animal as those of vegetative and those of animal life. Those functions which are carried on independently of the will, and are not influenced directly by changes in the external world—digestion, assimilation, secretion, circulation, and reproduction, for example—are called the vegetative functions of the body; while those of relation, such as sensation and voluntary motion, are called, in contradistinction, the functions of animal life.

The difference which has led to the general acceptance and current use of these and many similar expressions is real, as far as the higher and more familiar animals and plants are concerned, but, with the growth of our knowledge of the lower forms of life, the necessity for expansion and modification of the definition of both groups becomes apparent. Many of the lower animals, such as the hydroids and sponges, as well as many highly-organized animals, like the tunicates and oysters, lack the power of locomotion; and, on the other hand, many of the lower plants are quite actively locomotive. Certain plants which are by no means low are quite sensitive to external changes, and the actions by which they respond to these changes in the Venus's-flytrap, for instance, give quite as good proofs of the existence of volition as are afforded by the actions of many of the lower animals.

Another superficial and easily-recognized distinction between animals and plants is afforded by the contrast in general form and structure; this, like the first, is real, as long as our attention is restricted to the higher forms of the two groups. In the animal we find a sharply