Popular Science Monthly/Volume 44/January 1894/Logical Method in Biology
|LOGICAL METHOD IN BIOLOGY.|
THE logical processes involved in scientific reasoning are the same in kind as those used in the everyday life of the masses. The difference between the two lies in the clearer recognition of the processes and their importance in the scientific field. There is nothing like exactness in the applied logic of everyday life, and the reasoning of science is superior to the "common sense" of mankind only in being more exact. In science the comparatively little work that survives and does not have to be done over and over owes its superiority to this same exactness. Science has no peculiar method of its own either of discovering facts or of treating them.
Scientific students spend little time on the consideration of logical processes, because the mind follows them instinctively; and the study of them, for practical purposes, seems to be superfluous. But apart from the fact that they present a set of phenomena as worthy of scientific treatment as the phenomena of light or of the molluscan nervous system, it is important to consider them because of their direct bearing on every department of science. Even the best established sciences have reached their present states by successive approximations toward exactness, by the gradual elimination of errors of both fact and method; and even the novice knows that the degree of confidence placed in the statements of fact of a scientist by his contemporaries or succeeding generations depends directly on their confidence in his method. The history of any biological problem will furnish material for a comparison of methods. The present state of the problem will be found to owe its superiority over any earlier stage not simply to the greater number of facts that can be brought to bear upon its solution, but chiefly to more exhaustive methods for the discovery of new facts and interpretation of old ones.
There are comparatively few models that will serve as illustrations of the applied logic of the sciences, or of a sound and complete scientific method. Apart from quantitative exactness, the problems of biology can be given the same rigid application of logical principles as any other science; and in recent years much progress has been made toward giving numerical expression to both facts and laws in biology. The two following examples of scientific method—the one from experimental physiology, and the other from invertebrate morphology—show themselves, on analysis, to be models of vigorous generalization and deductive inference, prompt verification, reinterpretation of old facts, explanations of old contradictions, and removal of old obstacles to a clear understanding of the matters in question.
Fifty years ago Arnold discovered that the iris of the eel's eye contracts, producing contraction of the pupil, on being exposed to light after the eye is cut out of the head, and even when the anterior part with the iris is separated from the posterior part of the eye; but that when the outer or ciliary rim of the iris is cut away no reaction follows. It seemed to be conclusively proved that in the production of the phenomenon light acts directly on the ciliary part of the iris. A few years later Brown-Séquard discovered the same reaction in the frog's eye, and inferred that the light acts directly on the muscle elements of the iris. This inference he left entirely without verification, and even asked himself, without trying to answer, the question why, if light acts directly on the muscle fibers of the iris, it does not act thus on the other muscles of the body. In 1854-'55 Budge, after apparently exhaustive experiments, denied that the pupil of the excised eye contracts when light falls on the iris and not on the retina. In 1859 Müller proved that light acts directly not only on the outer rim of the iris, but more intensely on the inner or pupillary part. After a lull of twenty years in the dispute, Edgren proved that after destruction of the retina there is no reaction at all, and that therefore light does not act directly on the iris. The only fact that remained undisputed in this strife of fifty years was that the pupil of the excised but otherwise unmutilated eye of frogs and eels contracts when the eye is exposed to the light. It was still unproved whether the phenomenon is due to an intraocular reflex which, involves the retina or to direct action of light on the iris. The latter view was the favorite one, but no efforts were made to characterize the elements of the iris on which the light acts, and so to clear up the physiological conditions of the phenomenon.
"Stick to the facts!" impetuously shriek many biologists when some luckless fellow insists that scientific method, with the principles of logic at its foundation, requires careful and incessant attention in biological investigation. In this case the sequel shows that there can be no doubt of the truth of any of the statements made during the whole fifty years. But vociferous discussion could not remove the contradictions, and experiment only multiplied them. Confusion is as consistent with facts as harmony is. It was the lack of a clear logical analysis of all the conditions of the problem that led to the contradictions. Nothing was demonstrated until these were removed, and it is an important fact that they were finally removed, not by disputing them, but by reproducing the conditions of the contradictory experiments and incorporating the contradictions themselves into the final solution of the problem.
Steinach, by his recent experiments, demonstrated that the sensitiveness of the iris varies immensely in different individuals of the same species; that the iris of frogs, kept for days in glass cases, does not respond at all to alternate shading and exposure to diffused daylight, but slightly to concentrated gaslight, and gives a regular reaction of appreciable amount only on exposure to concentrated sunlight; that when frogs are kept for a long time in the dark the iris responds promptly to diffused daylight; but if, after the light has produced contraction of the pupil, the frog, instead of being put back in the dark, is left exposed to the light, the pupil gradually dilates in spite of the light, and after some hours acquires a state of comparative insensibility, so that moderate changes in the light produce no changes at all in the iris; and that the difference in pupillary reaction between frogs kept in the dark and frogs exposed continuously to light is greater in the excised than in the normal eye, greater still when the iris is isolated from the rest of the eye, and that, while in frogs of medium excitability of iris the isolated eye still responds to light after shading, the iris, when separated from the rest of the eye, no longer responds even to the strongest light. One would think that at least some of these preliminary conditions of success would have thrust themselves upon the attention of the earlier investigators if they were not altogether lacking in the qualifications of true scientists. They were probably no more lacking in analytical power than any other set of investigators, but they had certainly not made the most of it. Steinach exhausted the conditions by taking into account, one at a time, the different degrees of inherent excitability, effect of exposure to and exclusion from light, and the reactions of the normal eye, the excised eye, and the isolated iris, against the different degrees of light, thus:
|Normal eye.||Concentrated gaslight.|
|Excluded from light.||Concentrated sunlight.|
|Frog's eye.||High excitability.||Isolated iris.|
These do not include all the conditions which he detected, but they are sufficient to indicate the difference between his method and that of his predecessors. The modifying conditions were not discovered in the order in which they appear in the table, but tabulation shows very quickly whether or not they have been exhausted.
When all the favorable conditions were combined there invariably resulted a characteristic contraction of the pupil, on exposure to light, whether the object experimented on was the normal eye, the excised eye, the isolated iris, or the isolated iris deprived of its ciliary rim. In other words, the contraction of the pupil in the excised eye of fishes and amphibia does not depend on an intraocular reflex involving the retina, but on the direct influence of light on one or more of the elements of the inner or pupillary part of the iris. It had been suggested that the phenomenon was due to the action of light on the endings of the nerve fibers in the sphincter muscle of the iris. Steinach removed this suggestion from the group of remaining possibilities by paralyzing the nerves of one eye of an animal with atropine and leaving the other normal, and showing by comparative tests that the two eyes continue to act alike. He showed by a special experiment that the posterior pigment layer of the iris has nothing to do with its contraction. The branched or stellate pigment-cells—the chromatophores—in the front part of the iris were possible factors in the problem. They were known to undergo changes due to the action of light. Light causes a redistribution of the pigment within the cell, causing it to collect at the center. When the eye of an animal which has been kept in the dark is alternately shaded and exposed to the light, there follow a prompt alternate dilatation and contraction of the pupil. This process can be carried on for some time before there is any visible change in the chromatophores; at the end of half an hour or more the chromatophores are "contracted" but the pupil contracts and dilates as before. Therefore the contraction of the iris is independent of the changes in the chromatophores.
At this point physiological experiment had to be abandoned, and it would have been extremely comfortable for Steinach to do as one of his predecessors had done—ride the rest of the way on a cantering hypothesis; but he appealed to histology. In his effort to determine in what other parts of the iris there was pigment, through which the light must produce its effect, he found that his judgment was confused by particles of pigment from the posterior layer, which were scattered at random over his histological preparations. This difficulty was obviated by removing the posterior layer of pigment before making the sections. After taking this precaution he showed that there is no ordinary pigment in the stroma of the iris; neither are there any ordinary smooth muscle fibers like those in the iris of the higher vertebrates. He found the sphincter muscle of the iris composed of spindle-shaped pigmented cells. That these are really muscle fibers he proved by their form, size, characteristic fibrillar structure, and function. It was impossible to observe directly the contraction of these fibers; he adopted the indirect method of killing the iris in the relaxed and in the contracted states and observing the condition of the fibers in each. In the former they were slender and narrow, in the latter shorter and thicker. The ciliary muscle fibers are not pigmented, and this accounts for their being indifferent to the light. His general conclusion is that light produces contraction of the isolated fish and amphibian iris by acting directly on the fibers of the sphincter muscle through their pigment.
The striking characteristic of this investigation is the exhaustive consideration and removal of alternative beliefs. His final conclusion is only an inference, and derives its "certainty" from the fact that it is the only belief that is left. In its relation to this conclusion the evidence is circumstantial. If now the reaction of the pigment and fibers could be directly observed, Steinach's conclusion would be set down as a verified prediction. Though unverified, it is unhesitatingly accepted, like so much of our "knowledge," as an important truth; for most minds its verification would add little or nothing to its certainty, and would even deprive it of some of its interest. This inferential knowledge forms a large part of scientific truth, and other instances of it will appear in the following example of method in morphology.
Various ciliated organs of unknown function in different mollusks had never been brought under the yoke of homology. One of the most decisive tests in morphology for the determination of homology is the mode of innervation of an organ. Spengel reasoned that the homologies of these organs could be best established by a comparative study of their modes of innervation—in other words, by discovering their relations to other organs known to be correlated in definite ways among themselves. In this way he succeeded in proving their morphological identity, although the belief that they are olfactory organs is based simply on the morphological fact that they invariably occupy a certain position in relation to the respiratory organs, and not on any physiological data.
He demonstrated the general occurrence of this particular kind of organ in the prosobranch gastropods, inferred that it ought to occur among the opisthobranchs, and succeeded in demonstrating its presence in the division of tectibranchs. He had already in his possession the hypothesis that the organ is one belonging to the mollusca as a whole and drew from it the deduction that it ought to be present in the lamellibranchs, among which it had not been hitherto known. He said: "The position in which such a one would have to be sought was clearly enough indicated to me by my observations on the gastropods. It would have to be in the neighborhood of one of the ganglia of the visceral commissure." Trusting this definite anticipation, he looked for the olfactory organ and found it in Arca Noæ, the first mussel he opened for the purpose. In this species the organ is characterized by pigment, which made its recognition easy. In other species that he examined the pigment is absent, and had he first opened one of these, he might have had a long and possibly fruitless hunt for the organ. This well illustrates how important a part chance frequently plays even in deductive investigation. It is interesting to note how the deduction might have remained unverified and possibly have beenand yet have been a true one.
The organ typically consists of thickened epithelium innervated from a ganglion underlying it. Theory required the presence of a ganglion under the olfactory organ of lamellibranchs, but there was apparently only a strong nerve, which had hitherto been universally interpreted as the "gill nerve." Histological examination proved it to be an elongated ganglion inserted on the nerve between its origin and its ending in the gill. Here again, a deduction led to a discovery and the correction of what had seemed for years to be a settled fact.
Spengel had shown, in his study of other groups, that the nerve on which the olfactory ganglion lies arises from the visceral ganglia inserted in the visceral commissure, and not from the pleural ganglia. With one stroke of deduction he swept away a whole brood of old views. He reasoned that the parietosplanchnic ganglia of lamellibranchs, from which this nerve arises, must on account of this very fact be the visceral ganglia, and not what they had been universally assumed to be—the homologues of the pleural ganglia of gastropods. The old view necessitated the belief that the renal, reproductive organs, etc., of lamellibranchs are innervated from the pleural ganglia, and that the foot with all its accessories is included within the œsophageal ring of ganglia; whereas in other mollusks the renal, reproductive, and associated organs are innervated from the visceral ganglia and the foot lies outside of the œsophageal ring. If the parieto-splanchnic ganglia of lamellibranchs are homologous with the visceral ganglia of other mollusca all the above-mentioned organs hold the same relations in lamellibranchs as in the other groups. This reinterpretation of so many known facts harmonizes the lamellibranch type completely with that of the general molluscan type and marks a distinct step in the progress of molluscan morphology. He pursued a similar though less complete course with the cephalopods.
By this method of morphological reasoning, accompanied and corroborated or corrected at every step by morphological investigation, a heterogeneous mass of facts was bound together under the principle of homology, and many new ones were discovered that would not have been brought into notice in any other way. Indeed, the principle of homology, together with the principle on which it depends, the correlation of organs, furnishes a basis without which it would be nearly impossible to make intelligent search for new facts. Incessant use is made of the general logical principle that things that are similar in some respects, are likely to prove similar in other and unknown respects, and that things similar in many respects are likely to prove similar in most or all respects, in anticipating biological facts. It is well known that many of the facts of greatest theoretical importance in biology have been overlooked until hypothesis pointed them out. Yet this power of prevision is one of the most dangerous of pitfalls. No rule can be laid down for the use of the principle, because there is none. There is a general precaution to be observed: similarity in a few respects is no warrant for inferring similarity in many respects, much less all respects. Too many biologists, among them some of the most eminent, seem to have a wrong conception of the function of this logical principle. Scholastic methods are the favorite butt of scientific wit, but that notorious old tendency to speculate without due regard to facts is not dead but only facing in another direction. The stupid blunders and worthless "results" due to it are charged up against the far-reaching logical principle itself and have given rise to a counter tendency that is no more creditable. The old cry, "Stick to the facts!" simply means that the danger of going wrong increases very rapidly as one passes by inference beyond known facts, especially when these are few in number. Perhaps the greatest boon that could fall to biological science would be such a thorough study of the history of the science by its own votaries that they would learn beyond the power of forgetting the fact that speculation alone is worse than useless, and that reasoning with verification is indispensable.
- Investigations on the Comparative Physiology of the Iris. H. Pflüger's Archiv für Physiologie, vol. lii.
- Die Geruchsorgane und das Nervensystem der Mollusken. Zeitschrift für wissenschaftliche Zoologie (April 22, 1881), vol. xxxv, pp. 333-383.