==History of the Living Machine.==--In the first place, we must notice that these machines have not been fashioned suddenly or rapidly, but have been the result of a very slow growth. They have had a history extending very far back into the past for a period of years which we can only indefinitely estimate, but certainly reaching into the millions. As we look over this past history in the light of our present knowledge we see that whatever have been the forces which have been concerned in the construction of these machines they have acted very slowly. It has taken centuries, and, indeed, thousands of years, to take the successive steps which have been necessary in this construction. Secondly, we notice that the machines have been built up step by step, one feature being added to another with the slowly progressing ages. Thirdly, we notice that in one respect this construction of the living machine by nature's processes has been different from our ordinary method of building machines. Our method of building puts the parts gradually into place in such a way that until the machine is finished it is incapable of performing its functions. The half-built engine is as useless and as powerless as so much crude iron. Its power of action only appears after the last part is fitted into place and the machine finished. But nature's process in machine building is different. Every step in the process, so far as we can trace it at least, has produced a complete machine. So far back as we can follow this history we find that at every point the machine was so complete as to be always endowed with motion and life activity. Nature's method has been to take simpler types of machines and slowly change them into more complicated ones without at any moment impairing their vigour. It is something as if the steam engine of Watt should be slowly changed by adding piece after piece until there was finally produced the modern quadruple expansion engine, but all this change being made upon the original engine without once stopping its motion. [Illustration: FIG. 45. A group of cells resulting from division, representing the first step in machine making.]