It is perhaps necessary to say, at the outset, that the history of the Evolution of man cannot be written as a plain, matter-of-fact tale. Many portions of this history are tolerably well understood, but there are other periods, in some of which notable steps of progress were made, of which no record has ever been discovered. We must therefore expect occasionally to be reduced to speculation, and here and there to meet with controversy and with opposing theories.
It is not proposed here to enter into any full discussion as to the origin of life. It may shortly be said that in the existing state of knowledge, no very definite theory is possible. We know that life is associated with a jelly-like or semi-fluid substance called protoplasm, which consists of a very complex mixture of albuminoids. These albuminoids are continually undergoing changes and interactions of a complex kind, the sum total of which constitutes life. Many of these reactions have been reproduced, or imitated, artificially, and have been shown to be purely chemical or physical. The chemical nature of the albuminoids is indeed so complex that some considerable time must yet elapse before it can be completely investigated; and until such time it is obvious that we cannot hope for any very definite conceptions as to the nature of life. Broadly, however, the majority of physiologists regard life as a highly intricate series of purely physical and chemical processes, and if such a view be accepted, there is no insuperable objection to a general theory of the origin of living from non-living matter. By this it is not intended to imply that the manufacture of living matter is an immediate possibility; for even according to such a theory as we have indicated, it would be supposed that living substance came into being by a very slow process of Evolution, which it is hardly conceivable could ever be repeated in the laboratory. Knowing, as we do, that there was a time when no life existed upon the earth, and believing, as there is good reason to believe, that there is no fundamental distinction between living processes and ordinary chemical and physical reactions, we may logically regard life itself as a product of a natural process of Evolution.
Fig. 16.—A typical cell (greatly magnified).
(k) Nucleus; (p) cell protoplasm.
Fig. 17.—The process of cell division.
c, The centrosome, the body which divides first, and which controls the division of the nucleus.