The fact of biological progress has struck many observers. Some have been content to believe that the single magic formula of “Natural Selection” would explain it adequately and without further trouble, forgetting that there must be at least some points of difference between a natural selection producing a degenerate type and natural selection leading to progress. Some biologists have lumped it, together with all other evolutionary processes which seem to show us a development along predetermined lines, under the head of _orthogenesis_--the (hypothetical!) tendency of organisms to unfold just one type of hidden potentiality. Bergson has been particularly struck with it: refuses to allow that it can have anything to do with Natural Selection or any determinist process, and ascribes it to his _élan vital_.
Here, as so often elsewhere, Bergson reveals himself as a good poet but a bad scientist. His intellectual vision of evolution as a fact, as something happening, something whole, to be apprehended in a unitary way--that is unsurpassed. He seems to see it as vividly as you or I might see a hundred yards race, holding its different incidents and movements all in his mind together to form one picture. But he then goes on to give a symbolic description of what he sees--and then thinks that his symbols will serve in place of analytic explanations. There _is_ an “urge of life”; and it is, as a matter of fact, urging life up the steps of progress. But to say that biological progress is explained by the _élan vital_ is to say that the movement of a train is “explained” by an _élan locomotif_ of the engine: it is to fall into the error, so often condemned in scientists by philosophers, and ridiculed in both by satirists, of hanging or at least disposing of a difficulty by giving it a long name.
Let us think of the condition of life on earth at any given moment of her evolution. Certain possibilities have been realized by her--others have not. To take a trenchant example, before the Carboniferous or thereabouts, the vertebrates had not realized their possibilities of terrestrial existence--nearly half the globe’s surface lay waiting to be colonized by backboned animals. The earth’s surface was conquered then--but the air remained unsubdued before the mid-Secondary. In every period, there must be not only actual gaps unfilled in the economy of nature--such and such an animal is without parasites, such and such a hot spring or salt lake is without tenants; but also improvements can be made in existing types of organization--a tapeworm could be more firmly attached, a salt-lake shrimp could tolerate an even higher concentration of brine.