Essays of a Biologist

by Julian Huxley

This increase has not been universal; many organisms have remained stationary or have even regressed; many have shown increase in one particular but not in others. But the upper level of these properties of living matter has been continually raised, their average has continually increased. It is to this increase, continuous during evolutionary time, in the average and especially in the upper level of these properties that, I venture to think, the term biological progress can be properly applied.

Used thus it is no more an a priori or an undefined concept. It is a name for a complicated set of actual phenomena, and if, with progress thus defined, we were to speak of a law of progress in evolution, we should be using the term law in a perfectly legitimate way, as denoting a generalization based on observed facts, and not as pre-supposing any vitalistic principle of perfectibility, any necessary and mysterious tendency of organisms to advance independently of circumstances.

The gas laws state that the pressure of a gas kept at constant volume increases in a particular way with increase of temperature. Now the pressure of a confined gas depends on the rate at which its particles bombard the walls in which they are contained, and the speed at which they are travelling. In a gas whose temperature is raised, many particles will, at any given moment, be travelling more slowly than the average rate when it was cooler, many even which had been travelling fast may now be travelling slowly. None the less, the average speed of all the particles is greater; and this and nothing else is what with perfect justification we sum up as our law.

In biological evolution, some organisms degenerate, some remain stationary, but the average of certain properties, and more especially their upper level, increases; and this tendency for certain properties to become more marked, this and nothing else, is what we sum up and generalize, again with perfect justification, as the law of biological progress.

The mechanism of biological progress demands a word: for it is noticeable that a mere fact, however well attested, makes a very different kind of impression from a fact explained and brought into relation with the rest of our knowledge. The impression is either less powerful; or else, an explanation being sought for, an erroneous one is found. It was Darwin’s great merit that, not content with the piling up of evidence in favour of the reality of Evolution, he at the same time advanced a theory which made it at least possible to understand how Evolution could have come to pass as a natural process. The effect was multiplicative on men’s minds, not merely additive, for facts are too bulky to be lugged about conveniently except on wheels of theory.

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.

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.