But differences in industrial and political conditions have produced different combinations and associations, so that Pekin is wonderfully unlike New York.  So in these early developing machines, quite a variety of method of organization was adopted by the different groups.  Now as soon as any special type of organization was adopted by any animal or plant, the principle of heredity transmitted the same kind of organization to its descendants, and there thus arose lines of descent differing from each other, each line having its own method of organization.  As we follow the history of each line the same thing is repeated.  We find that the representatives of each line again separate into groups, each of which has acquired some new type of organization, and there has thus been a constant divergence of these lines of descent in an indefinite number of directions.  The members of the different lines of descent all show a fundamental likeness with each other since they retain the fundamental characters of their common ancestor, but they show also the differences which they have themselves acquired.  And thus the process is repeated over and over again.  This history of the growth of these different machines has thus been one of divergence from common centres, and is to be diagrammatically expressed after the fashion of a branching tree.  The end of each branch represents the highest state of perfection to which each line has been carried.

One other point in this history must be noted.  As the development of the complication of the machine progressed the possibility of further progress has been constantly narrowed.  When the history of these machines began as a simple mass of cells, there was a possibility of an almost endless variety of methods of organization.  But as a distinct type of organization was adopted by one and another line of descendants all subsequent productions were limited through the law of heredity to the general line of organization adopted by their ancestors.  With each age the further growth of such machines must consist in the further development in the perfection of its parts, and not in the adoption of any new system of organization.  Hence it is that the history of the living machine has shown a tendency toward development along a few well-marked lines, and although this complication becomes greater, we still see the same fundamental scheme of organization running through the whole.  As the ages have progressed the machines have become more perfect in the adjustment of their parts, i.e., they have become more perfect machines, but the history has been simply that of perfecting the early machines rather than the production of new types.

==Evidence for this History.==—­As just outlined, we see that the living machines have been gradually brought into their present condition by a process which has been called organic evolution.  But we must pause for a moment to ask what is our evidence that such has been the history of the living machine.  The whole possibility of understanding living nature depends upon our accepting this history and finding an explanation of it.  At the outset we have the question of fact, and we must notice the grounds upon which we stand in assuming this history to be as outlined.