As we approach the position just foreshadowed — that the ultimate touchtone of moral behavior is how far it contributes to human evolution — the first question, temporarily put aside, becomes more pressing. It asks, "What is evolutionary advance?" In the narrower, technical, immediately-foreseeable sense of the level of advance of a contemporary society we shall come to this in Chapters 4,6,7, and 8. But first we do well to ask what it means against the total back-drop of what is understood today about organic evolution.
There is still much about the dynamics of evolution that the biologist does not understand. Why do some life forms exist with trivial change for millions of years, while others are constantly altering? Why do the ecological challenges of one environmental change produce spurts in evolutionary adaptation while other climatic or radiational changes simply wipe out dozens of species? Above all, what do we mean by evolutionary advance? That it is virtually impossible to handle "advance" by any single "straight line" continuous direction has been generally recognized since Morgan (1923) pointed to the phenomenon of completely new emergents — qualitatively different configurations — in evolution.
In some general sense — and if all forms of life are considered as a single growing "tree" — biologists have no doubt whatever that they witness a progressive evolution. But in any single species they will admit that evolution may stand still or even in some sense "regress." Pushed to define what is "higher" som biologists will defend only the proposition that the "more evolved" forms show "more elaborate differentiation of organs." Huxley, for example, talks of "novelty, greater variety, complexity and higher levels of activity." Except for the last, these are not question begging and might be placed on a fairly objective and even quantitative footing. But are they only later, or also better? Is the digestive system of a cow more "developed" than that of a human because it has extra parts, and, in mechanical evolution is a propeller plane more advanced than a jet, for the same reason? What is hidden in this concept of organ differentiation is surely the more fundamental consideration that the more elaborate apparatus generally produces adaptation to a wider range of circumstances and thus better chances of survival.
A sure way of avoiding value judgments — but perhaps some valuable quantitative scaling too — is to say that whatever fails to survive is less advanced than whatever survives. Besides offering nothing better than a dichotomy this has several weaknesses. From the standpoint of practical use in deciding between the levels of evolution of two living organisms it has the defect that it cannot be used until one is extinct! Secondly, what survives and thrives in one situation may not do well in another. Some very "lowly" organisms that have found a highly stable and unchallenging ecological niche have survived, unchanged, where more developed organisms in more exacting environments have perished. And mere length of survival will not do. We have a conviction that the axolotl, which has learnt to live in caves and has lost its sight, has in some way regressed relative to earlier forms. And because I abandon my snow boots in summer and take to sandals, are sandals a more complex structure?
Inevitably one has to come to a formula which includes an evolution of the environmental demands as well as of the performance of the creature. The child who gets top marks in the third grade is not as "evolved" as he who gets middle grades in the twelfth grade. To reduce the length of exploratory discussion let us tentatively conclude that evolutionary level must be assessed through several expressions, as follows, among which, however, the most fundamental is the breadth of the environmental demand that is met.
(1) Among species in the same environment, those which fail to survive may be judged inadequate in some respect compared with those that do.Let it be repeated that although the above reads initially in biological terms, yet by the time we come to man, with his complex communal living groups, it applies also to the cultures he has developed in those groups. For example, we may say that the more advanced culture is that which can survive, without breaking down, over comparatively severe changes in, and demands for new adaptation from its environment. Implicit in (4) and (5) is that where human societies are concerned part of their capacity to maintain insightful control of environment resides in their capacity to support individuals of higher levels of intelligence and education. Thus among men emphasis shifts to "degree of understanding and control of society's environment and itself" as the definition of the measure of advancement of a society.
(2) Although organ complexity and differentiation is not primary, it is sufficiently correlated with superior function to be given some weight. For if other organisms could achieve the same adjustment capacity with simpler structure they would have replaced the more complex.
(3) Granted equivalent terrain, food supply, parasites and predators, the more abundant is the more successful.
(4) Most fundamental in the definition is the complexity and breadth of the environment to which the organism can adapt. Since environments are liable to change, and more species than are now living have disappeared through failure to adjust to changes in environment, ability to survive when transplanted is a measure of level of evolution.
(5) The more advanced is that species which is more aware of (understanding of) and able to control its environment. Purely cognitive "adaptability to new situations" is one of the psychologist's best definitions of intelligence and refers to a capacity which is biologically one of the last to appear. Since such understanding and control is effective in ensuring survival across gross changes in environment (as in men on the moon), it come close to being synonymous with (4), but can be distinguished as a measure of how much change the organism produces in its environment.