Essays of a Biologist

by Julian Huxley

The process will take time, for, whatever theory of variation we may hold[8]—the old idea of small continuous variations; or that of large mutations big enough to produce new species at one jump; or the most probable theory of numerous small mutations—they one and all must grant that the largest variation occurring at one time in a living species is infinitesimal in comparison with the secular changes of evolution.

There will further be a premium upon progressive changes, since a progressive change will generally land its possessor in virgin soil, so to speak; if not in an actually new physical environment, then in a biologically new situation. The placental mammal occupies the same dry land as did the wonderful reptilian types of the Secondary epoch. But constant temperature and embryonic nutrition within its mother provide delicately adjusted conditions in the early phases of development which in their turn enabled a more elaborate and more delicately responding brain machinery to be constructed in development, and so advanced their possessors on to new shores of control and independence.

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.

What could be more striking than the parallel between the rise of the mammals to dominance over the reptiles, and the rise of the motor vehicle to dominance over that drawn by horses?

In both cases, a comparatively long period in which the new type is in a precarious and experimental stage, only just managing to exist, of small size and rare occurrence, and in no real sense a serious rival to its old-established competitors. Then, suddenly, a change. It reaches a level at which it can effectively compete with them. What happens? In the case both of man-made machine and evolving vertebrate group, there is first a sudden increase in numbers of the new, a corresponding decrease in numbers of the old type. The upper level of size of the new type also begins to increase, and it begins to split up into a great number of differentiated sub-types. Some of these sub-types become extinct, others, on the other hand, are gradually improved, while still others undergo such rapid change as to merit the style of new sub-types. The upper level of size, complexity, and efficiency increase, both in animal and machine.

It is as well to remember that survival-value means only what it says. A variation with survival-value helps its possessors to survive: it is not the best possible variation of the kind. In the developing motor-car, the substitution of four for one or two cylinders was a great improvement. It had “survival-value”; and not until the majority of cars came to be four-cylindered was the additional advantage of six or eight cylinders large enough to bring them into existence as dominant types.

To the interrelated evolution of carnivore and herbivore, again, leading to increase of size and speed in both, of wariness in one, of tooth and claw in the other, we have again a close parallel in the interrelated evolution of armour-plating and of projectiles. Here again the process is gradual. We can further see that the sudden “development” of full modern armour on the first iron-clad would have been actually disadvantageous, since it would have reduced its speed relatively to other less heavily protected ships, without conferring any corresponding benefit in the way of defence against the comparatively inefficient projectiles of the day. Only when the range and piercing power of the projectiles increased did increase of armour become imperative.

To resume our pressure analogy, the natural increase of all organisms leads to a “biological pressure.” So long as a species remains unchanged, so long must it stay subjected to the full force of this pressure. But if it changes in such a way that it can occupy a new niche in environment, it is expanding into a vacuum or a region of lower pressure. Natural increase soon fills this up to the same level of pressure, and conditions thus become favourable for expansion into new low-pressure areas previously out of reach of the normal range of variation. Variation towards such “low-pressure” regions may be progressive, retrogressive, or neutral: but it is obvious that at each stage of evolution there will always be a low-pressure fringe, representing a considerable fraction of the “low-pressure” area within the range of variability, the occupation of which would be biologically progressive.

Thus from the well-established biological premisses of (1) the tendency to geometrical increase with consequent struggle for existence, (2) some form of inherited variability, we can deduce as necessary consequence, not only the familiar but none the less fundamental fact of Natural Selection, but also the almost neglected fact that a certain fraction of the guiding force of Natural Selection will inevitably be pushing organisms into changes that are progressive.