Popular Science Monthly/Volume 42/January 1893/The Logic of Organic Evolution
IN his work on the Principles of Science, Jevons described with great clearness the logical phases of scientific theories and illustrated them by a wealth of instances drawn from the sciences of mathematics, physics, astronomy, and chemistry. While he accords to the theory of evolution an importance equal to that of any other theory, he says but little about its evidence or logical history. He practically leaves the biological sciences and geology untouched, except in the chapter on classification, where he says, in closing the subject: "Natural classification in the animal and vegetable kingdoms is a special problem, and . . . the particular methods and difficulties to which it gives rise are not those common to all cases of classification, as so many physicists have supposed. Genealogical resemblances are only a special case of resemblances in general."
The sciences of chemistry, physics, and astronomy, based as they are on mathematics, allow precise statement and accurate experiment. In geology and biology, on the other hand, the factors are so complex that these sciences take on the nature of historical sciences, with all the difficulties which such a statement implies. The difference can be easily illustrated. Certain perturbations of the planets indicated the presence of another one as yet unseen. The amount of the disturbances could be accurately determined. Adams and Le Verrier almost simultaneously predicted the presence of Neptune at a definite point in the heavens, and the prediction was verified by the immediate discovery of the planet. The human race must have appeared at a definite time in some definite part of the earth; but biological science lacks all the factors with which to parallel the case of Neptune by pointing out by a prediction the time and place of the appearance of the race. It knows that the event occurred, but must wait for accident to reveal the place and depend on the broadest generalizations to reveal even the relative age of man.
The utter lack of rigidity in the relations of living things puts quantitative statement almost entirely out of question; except that in some cases it is possible to work out a valuable system of averages. Individual beings can be measured, but the laws of biology can not be put in mathematical form; hence the lack of mathematical precision, without which the history of a science does not lend itself very easily to logical treatment. Evolution is pre-eminently a historical law, but its relations to the evidence for and against it are such that its logical history is both unique and inspiring. If it has passed through a definite series of logical phases similar to those through which the mathematico-physical sciences have passed, if it has fulfilled the same conditions and led to similar and equally brilliant results, its logical status is permanently fixed.
The cell doctrine, which now lies at the foundation of biological science, illustrates an important principle in the growth of theories. The significant points in its history, for the present purpose, are: 1. It took its earliest shape as a botanical theory, arising from a very small part of the facts that it was destined to explain. 2. Those facts were the most obtrusive of the whole group of facts to which they belong. The most highly wrought products of the forces involved are always first discovered, and thus it comes about that the facts which are most difficult to explain and which are farthest away from the point where Nature began its work, at first form the foundation of a scientific theory. Knowledge increases by backward from the specialized to the generalized, and the theory is perfected only after a complete series of facts has been secured in this way.
One of the results of this principle is that all scientific doctrines in which a historical arrangement of phenomena is involved must pass through what is aptly called the catastrophic stage. The geological doctrine current in the early part of the century, that there have been successive world-wide catastrophes followed by recreations, was perfectly natural in that stage of the science. Only the most obtrusive facts were known. The mountains were regarded as simple products instead of very complex accumulations of the effects of forces working steadily. When geology passed from catastrophe to continuity, it made its great permanent stride forward by providing itself with a key to all the facts that have since been discovered.
The theory of evolution has passed through all these developmental stages. The law itself was not recognized until long after formal relationships had been established, and its discovery was simply a recognition of the principle of continuity. Natural history began with species—the mountain-ranges of biology—and regarded them as simple facts instead of last terms in a long series. The breaches between species, as between mountains, was what made them striking. The evidence that has been destroyed played an important part in the early stages of this as of all other lines of scientific reasoning which are dependent on historical evidence. The more hidden and comparatively insignificant facts, the residuum which constitutes the difficulties of classification, here as everywhere, compelled recognition by forcing themselves in increasing numbers upon the attention of biologists. The effort to suppress them by the old theory gave place gradually to the effort to base the new idea of continuity upon them. This period of reversal in scientific activity and the accompanying rapid reinterpretation of both the old and new phenomena, while it is recognizable in the histories of many sciences, is probably most striking in the history of evolution.
Every hypothesis by its nature accords with the facts from which it sprang. But it is the weakness of all hypotheses, true and false alike, that they are at first based on only a small part of the facts, and these are nearly always the most unsafe, because they are, as has been shown, the most highly specialized. The true hypothesis has to pass successfully through the ordeal of assimilating large bodies of facts that are already known by observation apart from the hypothesis. The theory of evolution did this as thoroughly and perhaps more rapidly since Darwin's time than any other scientific theory. There is a vast number of illustrations of this, but a typical one will suffice. Anatomy had long ago established the presence of valves in human veins, and physiology assigned to them the only intelligible function—that of preventing the blood from flowing back toward the capillaries. Had they been distributed throughout the venous system, there would have been no problem; but they are present in some veins and absent in others. No law regulating their distribution could be assigned, and students of human anatomy had to learn their distribution by sheer force of memory. Here was a fine group of arbitrary facts established by empirical observation. Not only was there no law to explain their distribution—their actual arrangement was utterly irrational if it were true that they were intended to prevent the backward flow of blood. It was easy enough to understand, from the old view of creation, why there should be valves in the veins of the arms and legs; but it was stultifying to learn that the spinal, iliac, portal, and above all the inferior vena cava, the largest vein in the body carrying blood upward, are without valves. To make the facts and their functional explanation still more incongruous, there are valves in the intercostal veins, in which the blood flows horizontally; and in the thyroid and internal and external jugulars, in which the blood flows down hill. Valves and gravitation apparently had nothing to do with each other.
Dr. Clevenger was the first to explain this group of facts by an application of the theory of evolution. If the theory is true, man's ancestors were quadrupeds, and the time during which he has walked upright is insignificant compared with the time during which they walked on all fours. The structures developed in his ancestors and not yet modified to suit his new posture should be expected to hold anomalous relations. So far as known, the general distribution of valves in the veins is the same in man as in the mammals near him, and when he is placed back on all fours the arrangement of the valves is perfectly intelligible. The veins of the limbs, the jugular and intercostal veins, then carry blood upward; and the venæ cavæ and other valveless veins are horizontal and have no need of valves. Many important facts of a pathological nature are accounted for by the theory of imperfect adjustment of bodily structure and posture. This explanation of them is so striking that Clevenger irreverently suggests that the original sin of man may have been the act of getting up on his hind legs.
When a theory has thus assimilated all the groups of facts related to those from which it sprang and which are unintelligible without it, it has fulfilled the philosophical requirements of a true theory. But every great generalization opens more problems than it closes. This has been true in astronomy, physics, chemistry, and geology, and is true of biology. There are now numberless questions to be answered in biology which could not even be raised without the theory of descent. An illustration may be drawn from the case already cited. Some of the cephalic veins have no valves, but should have them if the explanation is true; the azygos vein has rudimentary valves, but does not need them in the quadrupedal state. These facts become new problems and require subsidiary explanations. By regarding some of the imperfect valves as obsolescent and others as nascent, some disappearing because they are no longer useful and others appearing where they are needed, the exceptions are mostly removed. Frequently such exceptions are not simply accounted for under the theory, but form some of its most striking proofs. Fruitfulness in furnishing problems for solution, instead of indicating weakness, proves the strength of a theory. Alchemy, the Ptolemaic astronomy, and the doctrine of special creation alike, could not lead to a thousandth part of the scientific activity that has followed in the wake of the theories that supplanted them, because they furnished no way of approach to the numberless special problems.
The irresistible power of a true theory rests in the end in the possibility which it opens of asking and answering questions by the deductive method. It might be supposed that a theory without a mathematical basis would exhibit comparatively little power of prediction, although its validity might never be questioned. Quantity of effect can not be measured, much less predicted, by deduction from theories susceptible only of historical treatment. All that can be expected is the power to indicate the presence or absence of things that are still unknown. But the laws of heredity, variation, correlation, etc., although still undefined and perhaps undefinable, furnish peculiar opportunities for brilliant deduction.
Paleontology would be a sorry science without the power of restoration afforded by the principle of correlation. Its fragments of bones and teeth and stumps and leaves would be almost absolutely worthless. But from the standpoint of logic this is as truly prediction by deduction from known laws as the minute predictions for the nautical almanac. Perfect heredity would place the principle on a basis of certainty. Any one character of ruminants indicated to Cuvier the presence of all the rest. But the generalized types of paleontology are transitional forms possessing combinations of ruminant characters, with others belonging to the carnivora, such as he never dreamed of. But the recognition of secular change in the correlations of organs, instead of weakening, has strengthened the possibility of anticipating unknown facts.
In recent years the progress from deduction to verification has been so rapid that frequently the latter has followed at the heels of the former, so that the element of time has hardly entered between them to make them both more striking. Many of the deductions from the theory of descent, afterward verified, are commonplaces to the scientist, but their logical force is not sufficiently emphasized when the nature of the evidence is considered.
The doctrine of descent required the belief that ruminants once had upper incisors and canines. The belief was made almost a certainty by the presence of partially developed fœtal teeth where they are absent after birth. The confident expectation was justified by the discovery of generalized ruminants with full sets of teeth. If man were descended from lower forms, an explanation was required for the absence of the os centrale as an independent bone from the human wrist, for it is almost constantly present in amphibians, reptiles, and mammals. Rosenberg looked for it in the human embryo and found it. Wiedersheim was moved to declare that this was one of the greatest triumphs that morphology, based on the theory of descent, had yet won. The same logical process was exemplified in the discovery of abdominal ribs in the human embryo.
As long ago as 1801 Blumenbach inferred from the configuration of its skull that ornithorhyncus laid eggs, and his deduction, based on the known correlation of characters, was verified by Caldwell's recent demonstration that the monotremes are egglayers. There was an unverified deduction that monotremes must at some time have possessed normal teeth; it was verified by the recent discovery of calcified teeth in monotreme embryos. "Thatcher showed in 1887 that fins of fishes are derived from two pairs of lateral folds. In a paper on the significance of bone structure Dwight commented on this and said that if evolution were true he could see no reason why no vertebrate had more than four limbs. He also said that he could see no reason why no vertebrate had more than two eyes. In the same year Watase demonstrated that the Japanese goldfish has eight limbs and Spencer showed that all vertebrates have a third eye in the pineal gland. Spencer predicted that fossils would be found in which the pineal eye was functional; Cope demonstrated in fossils the orbit of this third eye and pointed out the attachment of muscles for its movement."
"Evolution suggested the annelids as remote ancestors of the vertebrates. Kowalevsky was thus led to the discovery of germ-layers among these animals—homologous with those of vertebrates. In the same way, but on the other side, Semper was led to the discovery of genuine worm kidneys (nephridia) in the lower fishes. Starting with the theory of genetic affinity, we have discovered a whole series of organs and tissues that had hitherto remained unknown. For example, we have now the 'spinal ganglia' in worms, the 'spinal nerves,' and, what is perhaps more important, we have discovered the sense organs out of which the sense organs of the vertebrates have been gradually built up."
One of the best-defined anticipations, and one that found confident expression from different sources, in the way of special predictions, long before it was verified, was the belief that primitive mammals were generalized types. Cope wrote, March, 1874, "I trust that I have made it sufficiently obvious that the primitive genera of this division of mammals (Mammalia educabilia) must have been bunodonts with pentadactyl plantigrade feet." "No perissodactyl or artiodactyl mammal was known at that time to possess such feet, nor was any perissodactyl known to possess tubercular teeth." Since the prediction was made, Cope has described nine species of the Eocene genus Phenacodus, probably the most generalized mammal known." They had five toes on each foot and a bunodont dentition; with primitive ungulate characters went complete sets of unmodified teeth and foot bones.
Marsh's famous pedigree of the horse illustrates the same process. Both he and others clearly foresaw many of the results that he afterward worked out. A large part of the logical value of this restoration of the genealogy of the horse family arises from the fact that what is now established by investigation was once an unverified deduction.
Such verifications, especially in paleontology, are often due to accident. The predictions might, therefore, and often actually do remain unverified and practically unverifiable, because it is not known where to look for the evidence. Such unverified deductions are frequently scouted as absolutely worthless for the purposes of biological science. This attitude is not taken toward similar deductions in other sciences, for the single reason that there are no external reasons for combating the theories. Enumerating a number of such cases from physics and chemistry, Jevons adds, "To my mind, some of the most interesting truths in the whole range of science are those which have not been, and in many cases probably never can be, verified by trial."
The most famous of these unverified deductions in biology is that concerning the descent of man. The facts on which the deduction that man is descended from lower animals is based are derived from anatomy and embryology. The evidence is circumstantial; but one of the most brilliant of the predictions enumerated was based on this deduction and verified by embryology. It is only with respect to the paleontological evidence that the "deduction is unverified." But if the chain of missing links were absolutely complete, it would only be circumstantial evidence. The direct evidence is forever beyond reach, because when the race was born there was no scientist present to observe it.
This case serves well to illustrate the nature of the objections to the theory. The best theory of evidence of a historical nature has been worked out by the law courts. There, no amount of negative evidence has any value whatever in the face of even circumstantial evidence of a positive nature. The only way in which the accused one can shake off the implication is, to furnish positive evidence that some one else committed the crime, or that he was in some other definite place when it was committed. A careful consideration of the well-understood doctrine of alibi, which involves all negative evidence, might have prevented a generation of logical inconsequence in the discussion of man's origin. The fact that no so-called connecting links have been found can have no value whatever as evidence until it is shown that the whole earth has been searched and that there are none, and that no such fossils have ever been destroyed by natural processes. The deduction is justified by the evidence from other departments.
Agassiz, I believe, made the promise to furnish the positive evidence that no such fossils ever existed by showing that the geological series, at least so far as man is concerned, is complete; and hence that if they ever existed they should have been found. Death interfered with the fulfillment of the promise. He, like others, believed that man appeared at some definite place at some definite time in the world's history. Had he succeeded in proving the geological series complete, he would have caught, not others, but himself, in his logical toils. He first of all men would have been under obligation to show when and where man did appear, and that connecting links were not among the circumstances that immediately preceded his appearance.
The radical disappearance of objections to the theory before the introduction of new and especially the pertinacity of the old evidence is extremely interesting. There are imperfections in the evidence, many of which can never be removed. But the difficulties are not logical but practical; they are due to scientific ignorance. In every phase of its development the theory has fulfilled the conditions imposed upon it by logic, and repeated the history of other established scientific doctrines. At first superficial and catastrophic, but approaching through formality to Nature's path, biological science finally entered upon an explanation of its natural arrangements and formal laws. The theory of evolution itself passed from the condition of a simple induction to the explanation of vast numbers of facts that had been empirically discovered; opened new fields of investigation; led to the discovery of whole series of phenomena that had been previously overlooked; and gave rise to confident expectation frequently culminating in definite predictions subsequently verified by investigation—until, in the words of perhaps the foremost investigator in America, "we are in fact doing hardly anything else to-day than to verify the suggestions which evolution makes."
- Jevons, Principles of Science, p. 727.
- Sachs, History of Botany.
- Physiology and Psychology, Clevenger, pp. 38-46. American Naturalist, January, 1884.
- Piles, prolapsus uteri, inguinal hernia, etc.
- Trout of Yellowstone Park.
- For a fine illustration see Wallace, Geographical Distribution of Animals, vol. i, pp. 209-214.
- Wiedersheim, Lehrbuch der vergleichenden Anatomie der Wirbelthiere, p. 223.
- Journal of Academy of Natural Sciences, Philadelphia, March, 1874.
- Cope, American Naturalist, August, 1884.
- Marsh, American Journal of Science, March, 1874, and other papers in the same journal.
- Principles of Science, p. 548.