EVOLUTION, ORGANIC. At a time when the contro versy over evolution, after dying down almost to nothing, has suddenly flared up again in certain backward regions and back ward circles, it may be as well to state the main problems briefly and concisely before embarking on a detailed discussion.
The first point to realize is that there is really not one problem, but three problems. There is first the problem of the fact of evolution : has evolution occurred or has it not? Secondly, there is the problem of the method of evolution : if evolution has occurred, what is the machinery by which it works? And thirdly, there is the problem of the result of evolution : again granting that evolution has occurred, can we discover any general rules to describe the course it has pursued, or find any main trends in the process? The third is really the affair of general or philosophical biology and does not especially concern us here. Suffice it to say that there undoubtedly exist certain general evolutionary tendencies, such as that towards increased specialization, with consequent divergence and multiplication of types. Further, that there does exist a main trend in evolution, which consists in raising the upper level attained by life in regard to various qualities making for greater control over and independence of the environment ; this main trend is generally called biological progress.
But the other two problems are of immediate concern, and it is essential to keep them distinct if the issue is to remain clear. Un fortunately, this is precisely what has usually not been done by many anti-evolutionists. They find a weak spot in some widely held theory as to the method of evolution, and at once proceed to state that the whole idea of evolution is false—a procedure only to be explained as the result of weakness of logic or of intellectual conscience.
It is obviously a good deal simpler to be sure that some process exists and is a fact than to understand precisely how it works and what its underlying machinery may be; and we need not deny the fact because we have not discovered the machinery. As an obvious example, we are very far from understanding the physiological and chemical machinery of development, by which for instance a hen arises from an egg ; but that does not cause us to deny the fact that hens do develop from eggs.
It is the same with evolution. Although we know a good deal more about the method of evolution than did Darwin or any of his immediate successors, we are still in doubt as to a great many points. But we are not in the least doubt as to the fact of evolu tion. Among those who have gone into the evidence, that is to say among competent biologists and geologists, there is not a single one who is not convinced that evolution has occurred and is occurring.
The evidence by now is overwhelming. Details will be given in the body of the article. Here it must suffice to enumerate the main lines. There is the evidence from fossils, which is now in many cases so full and detailed as to constitute complete proof in itself. It is however supported by many other lines drawn from different fields. There is the evidence from vestigial ("rudimen tary") organs, which are of no direct use to their possessors, but useful in other animals of the same general construction. It is impossible to account for these on any theory of special creatiqn, but their existence is to be expected on the theory of evolution. The facts of embryology (q.v.) afford similar evidence; so do the facts concerning the common plan which is found to underlie the structure of whole groups of animals. The facts of geographical distribution (see ZOOLOGICAL REGIONS) are easy to explain on evolutionary principles, difficult or impossible on any other view that has so far been put forward. So are the facts concerning variation both in domestic animals and in nature, and especially the total impossibility of drawing any sharp line between indi vidual variations, local races, sub-species and species (q.v.) ; and further the continuity of life through the generations tells in favour of evolution.
Finally there is the pragmatic value of the evolution theory. The biologist in studying living things, finds that the idea of evolution works and helps him to interpret his facts and to dis cover new facts and principles; while no other theory so far put forward helps him at all. The idea of evolution is as important a biological tool as, for instance, the microscope.
When we come to the method of evolution, matters are very different, and no final conclusion can be reached until a great deal more experimental and observational work has been done. Details of the main rival theories, will be considered later. Here it will be useful to point out that all such theories must fall into one of four categories (or a combination of two or more of these). First we may suppose that the environment has a direct moulding effect upon the race and racial history; this view is the basis of most Lamarckian theories. Secondly, the environment may have no effect, direct or indirect, evolution occurring as the result of changes which take place in the hereditary constitution for reasons inherent in its own nature; evolution would then be predetermined internally. This is the basis of the more extreme theories which go by the name of orthogenesis. Thirdly, the environment may have an indirect effect, by favouring some of the changes which occur in the hereditary constitution, so helping them to be per petuated, and frowning on others to help their extinction. This is the basis of the theory of Natural Selection. The variations to be selected may arise either completely at random, or only in certain directions; and they may be "spontaneous," due only to the inherent nature of the hereditary constitution, or they may be caused by outer agencies acting on the constitution. They must, however, be sufficiently numerous and varied for the environment to exert a sifting action upon them. Fourthly, the environment may have an indirect effect, but in a very different way—through the organism's endeavouring to adapt itself to its environment, the effort then altering its structure, and the alteration in structure being finally transmitted, in whole or in part, to subsequent genera tions. This is the basis of the remainder of the Lamarckian theories not covered by our first alternative. Of these theories, the first and the fourth demand the inheritance of acquired characters (see LAMARCKISM) as a prerequisite to their working; modern research leads us to believe that neither Lamarckian view is true.
The other two do not need the inheritance of acquired charac ters to be workable. In the present state of knowledge, it seems probable that the third method, of natural selection, is the main agency of evolution, though determinate variation (theory No. 2) may play some part. The most important problem before the evolutionist to-day is to discover how heritable variations origi nate, and to what degree they fall short of occurring at random but are limited by the nature of the hereditary constitution in which they occur.
It is often asserted by irresponsible persons that "Darwinism is dead." This is very far from being the case. In so far as Darwinism was a reasoned assertion of the fact of evolution, it is much more firmly grounded to-day than it was in Darwin's own time, and every year brings fresh evidence in its support. Only in regard to the nature of the variations which are to be selected has the theory of evolution by Natural Selection suffered any important modification; in other respects it remains unshaken.
The idea of evolution has penetrated many other departments of thought. Anthropology and ethnology are permeated with it, and so are history and comparative religion. Modem psychology recognizes that the human mind is unintelligible without an evo lutionary background. The idea of evolution has re-emphasized our kinship with the animals; it has dethroned man from his posi tion as lord of creation ; but in place of the old idea of fixity it has given us the idea of the possible advancement of the human race, and of man as the trustee of future evolutionary progress.
(J. S. H.)