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There will thus be a constant biological pressure (to use a term which, though still symbolic, a mere analogy, is less misleading and question-begging than élan vital) tending to push some of life on to new levels of attainment, new steps in progress, because any variations in that direction will have selection value, a selection value above the ordinary. And the process will be a gradual one, because variations are not very large; so that life no more realizes all potentialities of progress at once than did the United States or any other new country receive a uniform population over all its extent as soon as it was discovered, but had its people move in from the coasts in a regular and orderly advance.

There are plenty of parallels from human affairs. Indeed, the evolutionist can often gain valuable light on his subject, on what one may call the economics of the process, by turning to study the development of human inventions and machines. There, although the ways in which variations arise, and the way they are transmitted, are different from those of organic evolution, yet the type of “pressure,” the perpetual struggle, and the advantage of certain kinds of variation therein—these are in essence really similar.