Page:A Text-book of Animal Physiology.djvu/32

2 of living things as such? By what barriers are the animate and inanimate worlds separated? To decide this, falls within the province of general biology.

Living things grow by interstitial additions of particles of matter derived from, without and transformed into their own substance, while inanimate bodies increase in size by superficial additions of matter over which they have no power of decomposition and recomposition so as to make them like themselves. Among lifeless objects, crystals approach nearest to living forms; but the crystal builds itself up only from material in solution of the same chemical composition as itself.

The chemical constitution of living objects is peculiar. Carbon, hydrogen, oxygen, and nitrogen are combined into a very complex whole or molecule, as protein; and, when in combination with a large proportion of water, constitute the basis of all life, animal and vegetable, known as protoplasm. Only living things can manufacture this substance, or even protein.

Again, in the very nature of the case, protoplasm is continually wasting by a process of oxidation, and being built up from simpler chemical forms. Carbon dioxide is an invariable product of this waste and oxidation, while the rest of the carbon, the hydrogen, oxygen, and nitrogen are given back to the inorganic kingdom in simpler forms of combination than those in which they exist in living beings. It will thus be evident that, while the flame of life continues to burn, there is constant chemical and physical change. Matter is being continuously taken from the world of things that are without life, transformed into living things, and then after a brief existence in that form returned to the source from which they were originally derived. It is true, all animals require their food in organized form—that is, they either feed on animal or plant forms; but the latter derive their nourishment from the soil and the atmosphere, so that the above statement is a scientific truth.

Another highly characteristic property of all living things is to be sought in their periodic changes and very limited duration. Every animal and plant, no matter what its rank in the scale of existence, begins in a simple form, passes through a series of changes of varying degrees of complexity, and finally declines and dies; which simply means that it rejoins the inanimate kingdom: it passes into another world to which it formerly belonged.

Living things alone give rise to living things; protoplasm