Science

A VALUABLE contribution to the so-called Darwinian theory has recently been made by Mr. Chauncey Wright, in a paper on “ The Use and Origin of the Arrangements of Leaves in Plants,” published among the Memoirs of the American Academy of Science and Arts. By a comparison of the various angular distances of leaves arranged about a stem, it is found that they may all be resolved into the general form of a continued fraction, of which the approximate value is such a number that any given power of it is equal to the difference between its two next lower powers ; for example, Kn = Kn—2 — Kn—1. On this arithmetical peculiarity depends the geometrical fact that, in such an arrangement, each new or higher leaf must be placed over the angular space between the two older and lower ones which are nearest in direction, so as to subdivide this space in the same ratio in which any two successive leaves divide the circumference. This arrangement is that which effects the most thorough distribution of leaves around the stem, so that each leaf secures for itself the most complete exposure to air and sunlight, besides obtaining the most ample “ elbow-room ” for expansion in the bud. There is no evidence that this arrangement is anywhere completely realized in nature any more than the cell of the honey-bee ever thoroughly realizes the definite form which it suggests to the geometrician as that which secures the greatest possible amount of available room with the least possible expenditure of building material. Nevertheless, as is also the case with the honey-cell, this ideal arrangement of leaves is approximately realized, with all needful accuracy, in many of the different arrangements actually found. Thus the ⅝ arrangement differs from the ideal K only by 0.007, and the 8/13 arrangement differs only 0.003.

Mr. Wright’s inquiry into the sources and uses of this arrangement admirably illustrates the difference in scientific character between the doctrine of types and the doctrine of adaptation, of which Mr. Darwin’s theory of natural selection is part and parcel. The position taken is well expressed in an excellent foot-note concerning the parallel case of the bee’s cells. The doctrine of types regards the perfect geometrical cell-form as something archetypal or pre-existing which serves as a model for the bee’s instinct to copy. Plainly this is a metaphysical hysteron proteron, or putting of the cart before the horse. According to the doctrine of adaptation, the bee’s cells are slowly perfected modifications of clumsier structures of similar general pattern, — such as are actually built by wasps, humblebees, and other hymenoptera, — and their gradual approach toward ideal perfection has been determined simply by the usefulness to the species of parsimony in building with hard-earned material. It is well known to naturalists that there are great differences in excellence of workmanship among hive-bees ; even in the same garden may be found cells of various degrees of perfection, none of which absolutely answer the requirements met by the geometer’s ideal cell. But in proportion as the bee builds with economy is he favored in the struggle for life ; so that the ideal type of cell is just that which long-continued natural selection must inevitably tend to produce, and — as Mr. Wright very keenly observes — “ whatever evidence there is that the bee’s instinct is determined toward the ideally perfect type of the honey-cell is directly convertible into proofs that it is so determined by these simple conveniences and utilities.” The type, therefore, is not a pre-existing pattern to which the phenomena are in some inscrutable way made to conform ; but it is the goal toward which the unceasing operation of the principle of utility tends ever to urge the progress of the phenomena.

We purposely choose this collateral illustration from the bee’s cell, because it is more generally intelligible, and admits of briefer summing up, than any illustrations which might he gathered from the main discussion concerning leaf arrangement. Mr. Wright’s very striking essay carries out the principle of utility just indicated in its application to the various systems of phyllotaxy ; seeking to show that those arrangements which correspond to different forms of his ideal fraction are directly useful, or have been directly useful, either to existing or to ancestral plants as affording the most favorable conditions for the absorption and elaboration of sap. The subject is not an easy one to make clear to the general reader, and the author’s style, excellent and attractive as it is for its brevity and fulness of meaning, nevertheless by virtue of these very qualities demands the closest attention. To those, however, who have had sufficient scientific training to follow its line of argument, this essay will commend itself as a fine specimen of original investigation, valuable both to science and to philosophy, of a kind always too seldom met with, and very rarely to be found in our own country.

As we are speaking of natural selection, it may be well to note Professor Shaler’s ingenious explanation of the rattlesnake’s rattle. The existence of this appendage has long been a puzzle to philosophical naturalists, and Darwinians have been repeatedly challenged to account for the formation or preservation by natural selection of an organ assumed to be injurious to the species. The difficulty has lain in the assumption, too hastily made, that the noise of the rattle must be prejudicial to the snake by forewarning its enemies or prey of its presence, and thus giving the enemies time for sudden attack, and allowing the prey to escape. On the theory of natural selection, the preservation of the species must entail the atrophy of such an organ, or, rather, must prevent its origination, unless the damage occasioned by it be more than compensated by some utility not hitherto detected. Professor Shaler’s hypothesis, however, suggests the possibility that this whole speculation is fundamentally erroneous. Far from being injurious to the snake, by serving to warn its prey, it seems likely that the rattle may be directly useful by serving as a decoy. Professor Shaler has observed that the peculiar sound of the rattle is a very close imitation of the note emitted by a certain cicada common in American forests frequented by rattlesnakes ; and, according to his ingenious suggestion, the bird, hearing the note and thinking to make a meal of the cicada, advances upon its own destruction, becoming the eaten instead of the eater. If this be true, we may have data here for explaining some of the alleged phenomena of fascination, so far as rattlesnakes are concerned ; and another case will be added to the numerous cases now on record in which animals have acquired, for utility’s sake, peculiarities characteristic of totally different creatures. An alternative explanation, how ever, has been offered, which is worthy of careful attention. The general law that animals are benefited by concealment — the law which so beautifully explains many of the chief features of animal coloring — has some important exceptions. In many cases, when an animal is especially noxious, it is advantageous to him to be conspicuous, that enemies may recognize him at a distance and keep away from him. Thus, while grasshoppers, moths, and butterflies (on the exposed under surfaces of their wings) are in nearly all cases so colored as best to escape notice, on the other hand, bees and wasps, which are protected by their stings, and many beetles, which are protected by a noxious taste or odor, are apt to be conspicuously colored. A still better example is that of the jet-black toad which Mr. Darwin saw in La Plata, which emitted a poisonous secretion, and which, as it crawled on the light-tinted sand, could not fail to be recognized by every passing creature as something to be avoided. Now a rattlesnake is unquestionably a very noxious animal, and so dangerous to its enemies that they will always do well to keep out of its way. Moreover, the deathwound inflicted by it, though usually very sure, is somewhat slow in operation ; so that in a fierce struggle it will often happen that its action is not prompt enough to preclude a return of compliments fatal to the snake. When a tiger tears open the jugular vein of his enemy, the enemy is placed hors du combat at once ; but when the rattlesnake has bitten, there is nothing to prevent the foe from employing his short remaining period of probation in tearing the serpent to pieces, Hence the rattlesnake must be peculiarly benefited by an apparatus which serves as a signal to warn enemies of his presence, and to keep them from attacking him. His more formidable enemies, belonging chiefly to the mammalian class, are certainly intelligent enough to profit by such warning and shun the danger ; and as it is plainly the snake’s object to avoid even a conflict, it is clear that he is helped less by his terrible bite than by his power of threatening a bite.

This explanation seems to us in principle quite sound; yet if we adopt it, there is nothing to prevent us from giving due weight also to Professor Shaler’s suggestion. The success with which the note of the cicada is counterfeited by the rattle is a point to be more fully determined by further careful observation. And if it turns out that the rattle fulfils the double purpose of alarming sundry animals that are hostile and of enticing sundry others that are good for food, it will not be the first case in which it has happened that a structure useful in one way has also become useful in another way. The question is an interesting one, and valuable, if only because it reminds us of the danger of reasoning too confidently in an a priori manner concerning points the due elucidation of which requires careful study of the details of the every-day life of animals. It is one of the great merits of the theory of natural selection, that it has directed so many naturalists, with eyes open, into this fruitful field of inquiry.