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. These two sorts of possibilities really overlap. For instance, an increased efficiency of vision must be an improvement in pre-existing structures and creatures; it also involves the conquest of new regions of environment, and so in a real sense the occupation of a new biological niche. In any case, the changes which would confer advantage in the struggle for existence may take place in any direction--with, or against, or at right angles to the stream of progress. By means of those which march with that stream, the upper level of life’s attainment is raised. But the struggle still goes on: and again, starting from this new condition, there will be variations in every direction which will have survival value, and some of these will be progressive; and so the upper level will be once more raised.