It is clear that the meaning of the (log's actions can never be learned by studying his body; for if the which we apprehend is embodied in any structure it must be in our own rather than that of the dog. The dog no doubt knows, just as we do, that in the natural course of events the attack is a sign of a disposition to do him further harm, and that he may arrest or by doing something on his own part to meet it and prepare for it: but we know. in the case of most organisms, only the response and not the con sciousness of it. The important point, however, is this: The kick is a sign of something that may follow, and the responsive actions are not the mechanical equivalent of the kick; for they are directed or adjusted, either with or without consciousness and reason, to an event of which it is only the forerunner or sign. The changes in the stone are the effect of the blow, while the changes in the dog are, in sonic way, the result of the past history of the (log and of his ancestors; for, all through this history, violent assaults have been associated with danger of further Vio lence. This difference is as wide as the difference between life and its absence; and the independ ence of biology as a science is due to its existence. It is what Herbert Spencer means by the dictum that life consists in the continuous adjustment between internal relations and external relations: and it is what Aristotle means by teaching that the essence of a living being is not what it is made of nor what it does, but why it does it.
A living hieing is a being that responds to the changes go on in the world around it, for life consists in the maintenance of adjustment be tween the changes that go on in the external order of nature and those that take place in the living body. Life is response to the established order of external nature; for the responsive actions of living beings are such that our own reason approves them as judicious and reasonable and likely to prove beneficial. This truth has often found expression in the statement that living things use the properties of the world around them for their own goial, or that of their species. The same thought may be expressed by the statement that life is the use of the natural language of signs, for cavil stimulus to a vital act is. in course of nature, a sign with a signif icance; and the act is itself a response to the flea of which, in course of nature, the stimulus is the sign.
Physical analysis resolves living beings into organs and tissues and cells and physiological units and organic and molecules and at Mils ; and it may some day enable us to con struet a living organism by the combination of the proper elements. Physiology resolves vital activity into 'functions,' and it may some day. express these functions in terms of motion; as may also the psychology which investigates the elementary constituents of mental 'faculty.' A living being is, no doubt, an organic compound, but, to the biologist. it is also something more.
It is a coiirdinated whole; a member of a species; a part of the sum of life; and a constituent of the universe, which would cease to be what it is if nature were different. The biologist finds in it no self-sustaining power and no reality that would endure if it were abstracted from the natu ral world of which it is a part. Surely, this is good sense and good science. No physiologist who studies the waste and repair of living bodies, no naturalist who knows living beings in their homes, no embryologist who studies the influence of external conditions upon development, can for an instant admit that living beings are self sufficient or self-sustaining, or that their being is in themselves. For the line we draw, for better study. between living beings and the external world, is not one that we find in nature. but one that we make for ourselves by abstraction and generalization.
The external world of a living thing is as mach a part of it as its histological structure. It the environment of its body or of any cell or molecule NI it hill its body were different, neither cell nor body would be what it is; and if they bad no environment they would not be at all, for neither cells nor eggs nor seeds nor desiccated rotifers exist abstractedly. A self-sullieient and self-contained living being is as fabulous as a griffin or a centaur, but no naturalist thinks for an instant that this truth casts any doubt upon the real existence of living beings. It is not the real existence, but the abstract existence of living things, that the cautious biologist must dispute.
May not the truth that biology is the physics and ehemistry of living beings be true only when it is joined with the converse truth that physics and chemistry are part of biology? It is only as living beings that we care to know. If the modest biologist were to assert that the biologi cal aspects of the physical sciences are the only basis for rational interest in these sciences, his good friends in physical and chemical labora tories would no doubt charge him with arro gance, although they Must admit that the prin ciples of science, as distinguished from the con crete sciences, are part of biology.
We cannot investigate response to the order of nature without asking what the order of nature is. What are the properties of things and of thought that convince us of its existence? What is this conviction worth? What are the methods which knowledge of this order is acquired and perfected and extended? flow far are these methods and instruments trustworthy? Are any limits to their application known. and, if so, how known? To all these questions the biologist has peeuliar right to ask answers, in addition to the right which he shares With other students of science. The inquiry into the origin of those use ful properties in the employment of which life consists is the most faseinating and instrnetive intellectual oeeupation within the windy range of human inquiry, for to it knowledge itself owes its significance.