Popular Science Monthly/Volume 10/November 1876/What American Zoologists Have Done for Evolution: 1876 Speech I
|WHAT AMERICAN ZOÖLOGISTS HAVE DONE FOR EVOLUTION.|
IT would be pleasant indeed if only a lecture or an essay were expected from the presiding officer of the Section; but an address implies a great deal more, and the giver of it is not only expected to be entertaining, where perhaps he never entertained before, but instructive upon grounds upon which, perchance, he has made but partial survey. Among the many questions of sustaining interest, a number of subjects intrude themselves. A general review of the work accomplished since the last meeting of the Association would seem an appropriate subject for discourse. Yet beyond my special studies I feel quite incompetent to scan so broad a field. In this year of Centennial reviews, one might naturally fall into an attempt to sketch the growth of science and the work accomplished within the last hundred years, but that would not only be too vast a field, but would on the whole be unprofitable, since time-boundaries, like the surveyor's lines bordering a State, have no definite existence in Nature. The natural boundaries of oceans and sierras do indeed isolate and impress peculiarities of thought and action upon man, as upon the creatures below him, and for this reason we may with propriety examine the work of our nation in any line of investigation. Never before has the study of animals been raised to so high a dignity as at present. While chemistry could point to its triumphs in the arts, and geology to the revelations of hidden wealth in the rocks, zoology was for the most part a mere adjunct to geology, or a means to thwart the ravages of insects. Now, however, it is the pivot on which the doctrine of man's origin hinges. The worlds themselves are too old to study, though the spectroscope reveals the existence of celestial protoplasm as their physical basis. The rocks are too rigid and the time too immense to come within the compass of our minds, but the living facts of evolution are with us to-day in these graceful forms and their constant changes, while the records more or less preserved in past times give us a clew to things hinted at in the earlier changes of present existing forms. It seems, therefore, at the present time, that a review of the work accomplished by American students for the doctrines of natural selection might be acceptable for several reasons, and first among them might be mentioned the fact that thus far no general review of the kind has been made; and, secondly, that with few exceptions all the general works upon the subject are from English or German sources, and filled with the results of work done there oftentimes to the exclusion of work done elsewhere. The oft-repeated examples in support of the derivative theory belong to Europe. The public are familiar with these facts only, and come naturally to believe that these examples alone exist, and from their remoteness do not carry the weight of equally or perhaps more suggestive facts which lie concealed in the technical publication's of our own societies. A review of the work accomplished by American students bearing upon the doctrine of descent must of necessity be brief. Even a review of a moiety of the work is beyond the limits of an address of this nature. And for obvious reasons I must needs here restrict it to one branch of biology, namely, zoölogy. For material, the scientific publications of the country have been scanned, and an attempt has been made to bring together the more prominent facts bearing upon natural selection. In this review the zoölogical science of the country presents itself in two distinct periods: The first period, embracing as to time-limits the greatest portion, may be recognized as embracing the lowest stages of the science; it included among others a class of men who busied themselves in taking an inventory of the animals of the. country, an important and necessary work to be compared to that of the hewers and diggers who first settle a new country, but in their work demanding no deep knowledge or breadth of view. And so the work to be done in tabulating the animals has more often been done by specialists who neither knew nor cared to know the facts lying beyond the limits of their studies; a work often prompted by the same spirit that one sees among children in the collection of birds'-eggs and postage-stamps. The workers in this class were compared by Agassiz to those who make the brick and shape the stone for the edifice, an indispensable work, but with it was raised not the edifice but an almost insuperable barrier against the acceptance of views more in accordance with reason and common-sense. So thoroughly interwoven with this work were certain conceptions believed to be infallible, that overpowering indeed has been the argument to render as coadjutors the very men who so thoroughly opposed Darwin at the outset. It seems unnecessary to point out the mode of work adopted by the class above described. Their honor involved as soon as their name had been attached to a supposed new species, and any deviation from the type oftentimes persistently overlooked, what wonder, when every local variety received a new name and that name stamped upon a supposed valid creation—what wonder, I repeat, that whole groups of animals have been so thoroughly scourged by such work that few have the courage to engage in the task of revision?
Emerson's reflections on the science of England in 1847 would apply with far more propriety to our country even at a much later date, where in his words "one hermit finds this fact and another finds that, and lives and dies ignorant of its value." With the noble examples of Dana, Wyman, Leidy, and Burnett, before them, they did not profit. In fact, the labors of these honored men, and early in the century Lesueur and others, gave the country its largest claim to recognition abroad. The second period dates from the advent of Agassiz in this country. With his presence a gradual but entire change took place. He rendered the study a dignity rather than a pastime, No longer were the triflers to fling their loose work before the academies unrebuked. The protests he uttered in this Association were the means of elevating the tone of the communications. In fact, nothing indicates the poverty of our attainments in zoology more than an examination of the volumes preceding Agassiz's presence and the succeeding volumes. With his honest repudiation of all that was bad, he frightened away the lighter chaff, and there was but little solid work left to take its place. Agassiz made men, and his example, and the methods of work taught by him, spread to other parts of the country. He brought the American student into intimate acquaintance with the classical work of European naturalists. In his public lectures the names of Cuvier, Von Baer, Leuckart, and others, became familiar. The public caught the enthusiasm of this great teacher, and money was lavishly given by the citizens and the State in aid of his scientific undertakings. Agassiz's earnest protest against evolution checked the too hasty acceptance of this theory among American students. But even the weight of his powerful opposition could not long retard the gradual spread of Darwin's views; and now his own students,-last to yield, have, with hardly an exception, adopted the general view of derivation as opposed to that of special creation. The results of his protest have been beneficial in one sense. They have prompted the seeking of proofs in this country, and now our students are prepared to show the results of their work in evidence of the laws of progressive development, and it is mainly this work that I wish to review. So much is claimed for birthplace that, in the way of history, it may not be amiss to call attention to the fact that the first clear premonition of the theory of natural selection came from this country.
William Charles Wells, born in this country, at Charleston, South Carolina, in 1757, in a paper read before the Royal Society, in 1813, first substantially originated the theory to account for the black skin of the negro. He limits his application to races of men and certain peculiarities of color, correlated with an immunity from certain diseases; in proof of it he cites domesticated animals, and the selection by man in precisely the same line of argument urged by Darwin. In the preface to the last edition to the "Origin of Species," Darwin refers to Wells's essay as entitled to the credit of containing the earliest known recognition of the principle. Dr. Wells first shows that varieties among men as among animals are always occurring, and having cited the way in which man selects certain qualities among domesticated animals and thus secures different breeds, calls attention to the well-known fact that the black as well as the white races are differently affected by certain diseases of the countries which they inhabit. He finds a coincidence between the immunity from certain diseases and the black color of the skin, though why this is so he does not attempt to explain. He thinks that, through the successive survival of dark skins, the dark variety of the human race has become fixed. Referring to the man's selective action regarding domesticated animals, he says: "But what is here done by art seems to be done with equal efficacy, though more slowly, by Nature, in the formation of varieties of mankind fitted for the country which they inhabit." These sentences have such a Darwinian sound that, when we remember they were dragged from obscurity by Mr. Darwin himself, we can share in what a recent writer happily calls "Mr. Darwin's evident delight at discovering that some one else had said his good things before him, or has been on the verge of uttering them." As early as 1843, Prof. Haldeman discussed some of the arguments brought forward by the opponents of the Lamarckian theory, and offered certain views in favor of the transmutation of species. While he does not hint at the laws of natural selection, he recognizes fully the value of varieties and their persistency and ultimate divergence. He says, "Although we may not be able artificially to produce a change beyond a given point, it would be a hasty inference to suppose that a physical agent acting gradually for ages could not carry the variation a step or two farther, so that instead of the original one we will say four varieties, they might amount to six, the sixth being sufficiently unlike the earlier ones to induce a naturalist to consider it distinct."
In the year 1850, Dr. Joseph Leidy, in a paper on entophyta in living animals, wrote as follows: "The essential conditions of life are five in number, namely: a germ, nutritive matter, air, water, heat, the four latter undoubtedly existing in the interior of all animals." Dr. Leidy affirms his belief that very slight modifications of these essential conditions of life were sufficient to produce the vast variety of living beings upon the globe. The theory of derivation based upon the principles of natural selection demands the following admissions: that species vary, that peculiarities are transmitted or inherited, that a greater number of individuals perish than survive, and that the physical features of the earth are now and have been constantly changing, and that precisely the same conditions never recur. These are admitted facts. Now comes the theoretical part of natural selection, namely, that the varieties which survive are those which are more in harmony with the environments of the time. These propositions, with minor ones, form the theory of Darwin. Lamarck and others had recognized the gradual enhancement of varieties into species, but had not struck the key-note of natural selection, though Wells in the beginning of the century had clearly recognized it in a pertinent example. If we look impartially at these propositions, we need no demonstration to prove the inheritance of characters the most minute, and even the perpetuation of the most subtile features.
On general principles, too, the proposition, that those individuals best adapted to their surroundings survive, need only be stated to be accepted by a reasonable mind. In truth, to deny it would be to deny, as Alphonse de Candolle says, that a round stone would roll down-hill faster and farther than a flat one. Indeed, this eminent botanist affirms that natural selection is neither a theory nor an hypothesis, but the explanation of a necessary fact. The constant physical changes in the past and present condition of the world are incontrovertibly established. It seems, then, that the prime question resolves itself into whether each species as a whole has something inherent which prompts it to vary irrespective of its environments, or whether a correlation can be established between the variation of species and certain physical conditions inducing these variations, and here let me add that of all groups of animals from species through genera to higher divisions, that group of individuals recognized as a species has the most tangible existence. And, as a proof of this, there need only be mentioned the fact that many naturalists, while regarding species as clearly distinct, have on the other hand looked upon classification as an artificial method to facilitate the study, and hence the innumerable schemes and the successive interpolation of subclasses, sub-orders, sub-families, and sub-genera, which simply circumscribed smaller proofs than had before been recognized.
The rapid multiplication of some of these groups has already formed a serious obstacle to the study of systematic zoölogy.
What would good Dr. Mitchell have said if he could have foreseen the generic lists of to-day! In an article on the "Proteus of Lake Erie," he expressed his aversion to multiplying names in zoölogy, and lamented the tendency. He protested as follows, fifty years ago: "By some, these innovations have been so wantonly introduced, as almost to threaten in the end the erection of every species into a distinct genus." Though these words were undoubtedly aimed at Rafinesque, they were none the less prophetic. Whatever may be said of the existence in nature, of other groups, there can be no question that species have the most definite existence, and it would seem then that nothing more need be proved for the theory of descent as opposed to the theory of special creation, than the establishment of the fact that species assume the characters of new species, or disappear altogether with a change of surroundings. As examples might be cited, the transplanting of Alpine seeds to warmer regions below, and an accompanying change of the plant into another species before known in the warmer region, or, more remarkable still, the change of a species of Crustacean which lives in salt water, to another species with a partial freshening of the water, and this freshening slowly persisted in, the form changing into another genus, and in so doing losing: one of its segments. In the first case we see the effect of temperature, and in the second case the physical influence of salt and water in different proportions.
Now, these and hundreds of similar examples can be incontestably proved.
Even the prolonged existence of the form of some animals, like Lingula, may be referred to an inherent vitality which enables them to survive changes that caused the death of thousands of others.
In an early discussion of Darwin's theory, Prof. Agassiz cited the persistence of Lingula as fatal to the theory, and Prof. William B. Rogers replied that the vital characters of some animals would enable them to survive above others. Ten years later, I had an opportunity of studying living Lingula on the coast of North Carolina, and brought specimens home alive in a small jar of water, and kept them in a common bowl for six months without the slightest care. Their power of surviving under changed conditions—their vitality, in other words—seems incredible. (For further details, see reference below.)
It has for a long time been suspected that the species of Mollusca, described in such profusion in this country, would be reduced when the slightest attention to their habits had been made. Dr. James Lewis long ago observed that a certain species of fresh-water mussel, described as Alasmodonta truncata, is only the truncate form of another species, A. marginata. From a careful study of the conditions surrounding the first form in the Mohawk River, he had reason to believe that the rapid currents which pass over it bear along substances that, coming in contact with the exposed edges of the shell, break them down, thus retarding the growth of the shell at this point, and the animal concentrates its growth-powers to the repairs of the broken portion. The same gentleman also shows that the so-called species Lymnaæ elodes, catascopium, and marginata, "are modifications of one type or species, influenced by locality and temperature varying the method of development."
A. G. Wetherby calls attention to the variation in form of a group of fresh-water snails, found in the greatest abundance in certain streams of Tennessee and North Alabama. In showing the varied influences they are subjected to he cites the rapid currents of the channels, and the greater liability of the snails being torn from the rocks. He shows that they are exposed in various ways to the effects of these currents, with all their changing impetus of high and low water—exposed also to privation of food from the scouring sand removing the confervæ, upon which they subsist, from the rocks. He takes into account temperature, chemical action, and the like, and says, "No greater vicissitude can be imagined than this growth in an unstable element." Coincident with these diverse conditions he finds an enormous variety of forms, and frankly acknowledges that many of those described as distinct species must be reduced to synonyms.
George W. Tryon, in his large work on the American Melanians, published by the Smithsonian Institution, having finished his manuscript in 1865, says, under date of 1873, when the work was finally published, "A more enlarged acquaintance with fresh-water shells convinces me that a much greater reduction of the number of species than I have attempted must eventually be made."
If we now look upon the definition of a species, as given by a gentleman foremost in the ranks as a describer of species, we find it formulated as follows: A species represents "a primary established law, stamped with a persistent form (a type) pertaining solely to itself, with the power of successively reproducing the same form, and none other;" and this gentleman has not hesitated to base these "primary organic laws" upon the evidence of a single specimen, and in some cases even the fragments of one have offered him a sufficient inducement!
But it has been argued by some that a wide variation may be the case with many species. Prof. Agassiz, at a meeting of the American Academy, reiterated his opinion that what are called varieties by naturalists do not in reality exist as such. He found a great abundance of diverging forms in Echinoderms, which, without acquaintance with connecting ones, would be deemed distinct species, but he found they all passed insensibly into each other.
Prof. Parsons suggested that more extended observations might connect received species by intermediate forms, no less than so-called varieties; and Prof. Gray remarked that the intermediate forms, connecting by whatsoever numerous gradations the strongly divergent forms with that assumed as a type of a species, so far from disproving existence of varieties, would seem to furnish the best possible proof that these were varieties. Without the intermediate forms they would, it was said, be taken for species; their discovery reduced them to varieties, between which (according to the ordinary view), intermediate states were to be expected.
Recognizing, then, the existence of varieties, and of varieties sufficiently pronounced to have led careful naturalists to regard them as distinct species, what shall we say when it is found that these marked forms are correlated with certain physical conditions, many of which have originated within comparatively recent times? Dr. J. G. Cooper, after a careful study of the California land snails, ascertained that "species, sub-species, and varieties, living in cool, damp situations, become more highly developed (but not always larger) than the others; the shell assuming a more compact (imperforate) form, and losing those indications of immaturity referred to, viz., sharp, delicate sculpture, bristles, and angular periphery. These characteristics, however, remain more or less permanently for indefinite periods, and give that fixedness to the various forms, even when living under the same conditions, which enables us to retain them as sub-species differing from varieties in permanency, and from races in not inhabiting distinct regions." It may be added that Stearns, Bland, and Binney, have likewise observed the same peculiar variations associated with aridity.
In a broader field, and compassing different classes, Prof. Spencer F. Baird, Mr. J. A. Allen, and Mr. Robert Ridgway, have severally shown that marked and specific changes are seen in birds and mammals corresponding to differences in their surroundings. Prof. Baird, in a paper entitled "The Distribution and Migration of North American Birds," has shown that birds in high altitudes and those bred at the North are larger than those born South and at low altitudes; that Western birds of the same species have longer tails than eastern examples, and that the bill increases in size in those birds occurring in Florida as compared with those found north of that State, and that on the Pacific coast the birds are darker in color than those found in the interior.
Mr. J. A. Allen has made a more special study of this matter, and his work ranks among the most important contributions to this science. Mr. Allen finds that there are marked geographical variations in mammals and. birds. He shows that northern mammals of the same species are more thickly and softly furred, and that toward the south the peripheral parts, such as the ears and feet, are more developed. The same law holds good in birds, a diminution in size being observed toward the south, and the individuals being darker in color.
As one goes south he meets with the same species of birds, whose bodies are shorter, but whose beak, tail, and claws, are longer. On the Plains, also, he found the birds with plainer tints, while southward the colors became more intense. On drawing up a table indicating the regions of lighter varieties, and comparing it with a chart of mean annual rainfall, Mr. Allen found the lighter forms occurred in dry regions, and the dark forms in relatively humid regions. To sum up: Mr. Allen finds in latitudinal variation climatic influences affecting color as well as altering the size of bill, claw, and tail, while longitudinal variation usually affects color alone.
He states that these laws are now so well known that a species may be predicted to assume a given color if under certain specific climatic conditions.
Mr. Robert Ridgway has in a similar way called attention to the relation between color and geographical distribution in birds as exhibited in melanism and hyperchromatism, and has shown that red areas "spread" or enlarge their field in proportion as we trace certain species to the Pacific coast, and that in the same proportion yellow often intensifies in tint.
The results of these investigations can be easily understood. Nearly if not quite one hundred and fifty species of birds, which were recognized as distinct, are at once reduced to varieties, though less than twelve years ago they were looked upon as good species, with which no external influence had anything to do. Nearly if not quite a fifth of the number of species of birds have been reduced by the investigations of Baird, Allen, Coues, and Ridgway.
The mammals, through the same study of geographical variation, will have been reduced at least one-fourth. Already Mr. Allen has studied the geographical variation of the squirrels, and the result is that a reduction has been made of one-half the number of species before recognized. Prof. Baird, in his monograph of North American squirrels, reduced the number from twenty-four, as acknowledged by Audubon and Bachman, to ten well-established species and two doubtful varieties. Allen, with still greater advantage in the shape of a mass of material from the Western surveys, l-educed the ten species to five species, with seven geographical varieties.
Should it be urged that the present tendency toward reducing species be taken as an evidence that species had not before been properly defined, then it offers a stronger argument still in favor of the fact that species are even more variable than had before been supposed, leaving the greater possibility of larger numbers of these ultimately surviving. Again, the assumption that the limitation of specific variation had not been properly indicated, shows how reprehensible has been the work of some of those who have burdened our literature with their bad species.
The fact is, the work has in a measure been justifiable, and is not' to be wholly condemned. The workers in this line have followed the teachings of their masters. A group of individuals removed from an allied group of individuals by an extra dot or darker shade, perpetuating their kind from generation to generation, marked with persistent characters, and in every way coming up to the standard recognized as specific, had the right to be judged as such. It is only when a whole series of forms are collected, and climatic influences are seen to affect these in the same way that they affect other groups of species even in different classes, that the mere influence of moisture and temperature is shown to be the sole cause of many of these supposed specific characters.
Dr. A. S. Packard, in his remarkable monograph of a group of moths, the Phalænidæ, published under the auspices of the Hayden Survey, finds that with some species there are changes analogous to those pointed out by Baird and Allen; and while he does not find enough to establish a law, yet to his mind enough is seen "to illustrate how far climatic variation goes as a factor in producing primary differences in fauna? within the same zones of temperature," and he admits that varietal and even specific differences may arise from these climatic causes alone. Dr. Packard, in the same work, under the head of "Origin of Genera and Species," says, "The number of so called species tends to be reduced as our specimens and information increase." The genera also "are as artificial creations as species and varieties. The work of the systematic biologist often amounts to but little more than putting Nature in a strait-jacket."
An application of the influence of temperature is here proper, as explaining, on a rational ground, the persistence of peculiar arctic forms of animals and plants on the summits of Mount Washington and other high peaks. With a knowledge of glacial phenomena, we are capable of judging the condition of things which must, of necessity, have existed directly after the recedence of the great ice-sheet: its southern border slowly retreating, and, with the encroachment of the warmer zone, the arctic forms dying out, or surviving under changed conditions; but, in high plateaus and mountains, local glaciers flourished for a while, and at their bases arctic forms flourished, and, lingering too long, were ultimately cut off by the retreat of the main field. This interpretation of arctic forms on high peaks, though attended to by several American naturalists, is not new. Oswald Heer, in discussing the origin of certain animals and plants, coincides with De Candolle that Alpine plants are relics, as it were, of a glacial epoch. Prof. Gray had also independently arrived at the same conclusions, based on a comparison of the plants of Eastern North America and Japan. In the position he maintained regarding the derivation of species from preëxisting ones, he stood far in advance of his brother naturalists in this country, for this was before Darwin's great work had appeared, and before Heer had developed the host of fossil plants from the arctic zone. Mr. S. I. Smith, in speaking of mountain faunæ, points out the gradual encroachment of glaciers, and the drawing down of northern forms; and, as the glaciers retreated, these forms were caught, "the mountain-summits being left as aërial islands." Dr. Packard and Mr. Scudder have severally called attention to the same thing.
Prof. A. R. Grote has more fully dealt with the subject in a paper read before this Association, and in a graphic way shows that the "former existence of a long and widely-spread winter of years is offered in evidence through the frail brown Œneis butterflies, that live on the top of the mountains within the temperate zone." I have been thus explicit, in order to contrast these more rational views with those formerly entertained by eminent naturalists, whose minds were imbued at the time with the idea of special creation. Mr. Samuel H. Scudder read before the Boston Society of Natural History an account of distinct zones of life on high mountains, as illustrated in the insect-life of Mount Washing-ton. He called attention to certain insects which he supposed peculiar to the summit, and not found farther north, though showing a remarkable correspondence to certain arctic forms. Prof. Wyman asked whether all the facts might not be accounted for on the theory of migration northward after a glacial epoch, and Prof. Rogers suggested that the facts might be accounted for on the migratory theory if we added thereto the supposition of subsequent variation induced by isolation. Yet these views were persistently opposed by the other naturalists present. The mass of evidence already contributed, as to the extraordinary variation in color, markings, and size of species coinciding with their physical surroundings, though perhaps trivial in itself, becomes important when the proofs are grouped together, and all bear upon the theory of derivation. So slight a thins; as change of food is found to influence certain animals even to a degree usually regarded specific. The late Dr. B. D. Walsh discovered some very curious features among insects connected with a change of food. First, he established the fact that insects accustomed to one kind of plant could acquire a taste for another kind, and he has shown that in thus changing the food of the insect a change took place in the appearance of either the larva, pupa, or imago, and sometimes all three stages were affected. Dr. Fitch had observed that changing an insect's larva from the leaf to the fruit affected the appearance of the larva. It would be impossible to give even an abstract of Dr. Walsh's remarkable essay. It may be said, however, that his investigations led him irresistibly to the conclusion that the present species have been derived from preëxisting ones, and in numberless cases he is capable of showing the successive stages from the dawn of a plant-eating variety, where the changes are slightly seen in the larva only, to the plant-eating species in which profound changes are seen in the larva, pupa, and imago.
The minor factors of natural selection, such as protective coloring and mimicry, have been variously illustrated by Mr. R. E. C. Stearns, Dr. Kneeland, Prof. Cope, Dr. Charles C. Abbott, and others. In a special paper on "The Adaptive Coloration of Mollusca," I have endeavored to show not only a wide-spread application of this feature to mollusks, and especially those exposed by the tide, but in some cases a mimicry of inanimate objects, as the accumulation of clay or grains of sand upon the shell.
Wallace's theory of birds'-nests finds interesting confirmations in the observations of Dr. Abbott, who made a special study of a large number of robins'-nests, and found the widest variation among them. He studied also the nests of the Baltimore oriole, where, according to the theory of Wallace, a concealing nest should be made, the bird being exceedingly bright-colored. He found that, away from the habitations of man, the orioles built concealing nests; but in villages and cities, on the other hand, where they were in no special danger from predatory hawks, the nests were built comparatively open, so that the bird within was not concealed.
The differences in the habits of animals of the same species are noticed in different parts of the country, and such facts militate against the idea that certain unerring ways were implanted in them at the outset. Indeed, such facts go to show that these various creatures not only become adapted to their surroundings, but that individual peculiarities manifest themselves. The observations of Dr. Cones, Mr. Allen, and Mr. Martin Trippe, go to prove that certain birds change their habits in a marked degree. In their behavior, too, certain birds, which are wild and suspicious in New England, are comparatively tame in the West. In their resting-places they show wide individual variation.
Prof. A. E. Verrill, on the supposed eastern migration of the cliff-swallow, traces historically its first appearance in various places in the East, and is inclined to the opinion that as the country became settled by Europeans the birds left their native haunts for barns and houses, and increased in number to a greater extent than before on account of the protection invariably furnished by man.
Rev. Samuel Lockwood records a curious case of the Baltimore oriole acquiring a taste for the honey-sacs of bees, tearing off the heads of those insects, and, having secured the honey-sacs, rejecting the rest of the body.
Prof. Wyman observes a curious case in Florida of a colt and a number of pigs and cows thrusting their heads under water and feeding on the river-grass, in some cases remaining with their heads immersed for half a minute.
Hon. A. II. Morgan observes the widest difference in the habits of the same species of beaver in the Lake Superior region and in the Missouri, constructing their dams and ways differently, and meeting the varied conditions, not by a blind instinct, but by a definite intelligence manifested for definite purposes.
All of these facts, simple in themselves, yet together go to prove that animals do vary in their habits, and with a persistent change in habits arises the minute and almost insensible pressure to swerve and modify the animal.
So much does the influence of season, with its accompanying peculiarities of food, temperature, humidity, and the like, affect certain animals developing coincidently with its different phases, that it is instructive to note that in certain species of insects two or three different forms occur. Thus Mr. Edwards has in an elaborate way worked up the history of a polymorphic butterfly [Ephiclides ajax), showing that there are three forms heretofore regarded as distinct species, which are only varieties of one and the same species, but appearing at different times of the year, and consequently confronted by different influences as to temperature, moisture, food, and the like. These forms are known under the names of Walshii, Telemonides, and Marcellus, and both sexes are equally affected. The first form mentioned represents the early spring type, Telemonides the late spring type, and Marcellus the summer and autumn type (see also Mr. Scudder's paper). If these influences affect species, we should expect to see the greatest variety of forms in a country possessing the widest diversity of conditions.
Some suggestive paths of investigation have been pointed out by Prof. N. S. Shaler on the connection between the development of the life and the physical conditions of the several continents, showing first that the greatest amount of shore-line in proportion to the internal areas indicates a greater diversity of surface within.
Another proposition he attempts to establish: that in proportion to the shortness of the shore-lines, or, in other words, to the want of variety in their surfaces, will be the diversity of animal life in the continent. He then proceeds from Darwin's standpoint, and follows out many curious and instructive lines of thought regarding increased amount of influences in diversified surfaces—a level plain having the same conditions throughout, but a mountainous region having for each one thousand feet of elevation a new condition of things, in the form of streams, winds, humidity, and the like. In areas of simple outline and unvarying surfaces we do, in fact, have a less diversity of forms.
Recognizing the mutation of continents through past geologic ages, we again see the accompanying physical changes in not only modifying forms, but in selecting them afterward by succeeding changes.
The widely-diversified nature of the facts bearing on the doctrine of natural selection baffles all attempts at a systematic classification of them. Of such a nature are many of the valuable communications of Prof. Wilder.
At the meeting of this Association he has, among other matters, confirmed in a young lion the discovery of Prof. Flowers that, in the young dog and probably in other carnivora as well, the scapho-lunar bone has at the outset three centres of ossification, and that these really represent the radiale intermedium and centrale of the typical carpus. By study of a fœtal manatee, Prof. Wilder is able to determine its affinities, and to point out the probable retrograde metamorphosis of some ancient ungulate animal, and that the manatee is widely removed from the whales with which it has been associated.
Mr. William K. Brooks has published a very remarkable paper on certain free swimming tunicates, the Salpa, giving for the first time a clear and comprehensive history of certain obscure points, and has at the same time applied the principles of natural selection theoretically in showing the origin of salpa from sessile tunicates, and making clear the peculiar modification of parts which accompany these changes.
In the field of entomology some capital work has been done, both practical and theoretical.
Prof. Riley's demonstration of the yucca-moth is unique in its way. Dr. Engelmann has discovered that the yucca depends upon insects for fertilization; and Prof. Riley, by patient study, not only discovered the moth which fertilizes the flower, but finds an anomalous change in the maxillary palpi of the insect, by means of which the moth collects bundles of pollen, which it inserts into the stigmatic tube, and during this peculiar act deposits her eggs in the young fruit. Prof. Riley has reasons to believe that this is the only insect engaged in the fertilization of this plant. A mutual dependence is here met with of extreme interest. The yucca unfertilized forms no fruit, and the larva of the moth consequently perishes.
Prof. Augustus K, Grote, in an examination of butterflies, finds successive gradation in their structures, and shows that as these organs "become less serviceable to the insect they become more rigid and in position more elevated above the head in the butterfly, while in the moth they are more whip-like and directed forward." While protesting against the separations which have been made in the order based upon the antennæ, he directs attention "to the real differences in antennal structure between the butterflies and moths, while showing that the antennæ are modified by desuetude in the higher and former group." Prof. Grote, in dealing with a family of moths, the Noctuidæ, calls attention to the unequal value of Acronycta, and is forced to admit that these differences become clear through the theory of evolution. He says: "Where in Acronycta there is a general prevailing uniformity in the appearance in a single group of species and generally broad distinctions between the larval forms, it is a not unreasonable conclusion that these larval differences are gradually evolved by a natural protective law, which intensifies their characters in the direction in which they are serviceable to the continuance of the species."
Those who have believed in types as fixed laws, rigidly impressed at the outset of life, are those also who have recognized in the cells of a honey-bee, as well as in the arrangement of leaves about the axis of a plant, a perfect mathematical adjustment of parts, which were stamped at the beginning, and have so continued to exist without deviation. For nearly two hundred years it has been believed that the instinct of a bee guided it to shape a cell which of all other forms should use the least amount of material. A theory having been established as to the constant shape of a bee's cell, namely, that it was an hexagonal prism with trihedral bases, each face of the base being a rhomb with certain definite angles, a mathematician was given the problem to construct similar cells, and to determine the best possible form with the use of the least amount of material. The coincidence between theory and observation and experiment was so remarkable as to settle apparently for all time the question as to the perfectly-implanted instinct of the bee with its unconscious power of accurate work. Prof. Jeffries Wyman, to whose memoir I am indebted for the above facts, has by an ingenious study of the cells of bees shown, first, that, a cell of this perfection is rarely if ever attained. Furthermore, that, while the honey-cells "are built unequivocally in accordance with the hexagonal type, they exhibit a range of variation which almost defies description;" that the worker-bees, from incorrect alignment and other causes, build cells, the measurement of which shows the widest limit of variation; that the drone-cells are liable to substantially the same variations, while the transition-cells, namely, those in which drones and worker-cells are combined in the same piece of comb, are extremely irregular. As the drone-cells are one-fifth larger than worker-cells, "a transition cannot be made without some disturbance in the regularity of the structure." And Prof. Wyman states distinctly that the bees do not have any systematic method of making the change, adding that "the cell of the bee has not that strict conformity to geometrical accuracy claimed for it," and the assertion, like that of Lord Brougham, that there is in the cell of the bee "perfect agreement between theory and observation, in view of the analogies of Nature, is far more likely to be wrong than right, and his assertion in the case before us is certainly wrong." Prof. Wyman closes his essay by saying that "much error would have been avoided if those who have discussed the structure of the bee's cell had adopted the plan followed by Mr. Darwin, and studied the habits of the cell-making insects comparatively, beginning with the cells of the humble-bee, following with those of the wasps and hornets, then with those of the Mexican bees, and finally with those of the common hive-bee; in this way they would have found that, while there is a constant approach to the perfect form, they would at the same time have been prepared for the fact that even in the cell of the hive-bee perfection is not reached. The isolated study of anything in Nature is a fruitful source of error."
The remarkable ingenuity, so characteristic of Prof. Wyman's experiments, is fully shown in this memoir. He made plaster-casts of the comb, and then sawed transverse sections, and by slightly heating the plaster the wax was melted and absorbed, leaving the delicate interspaces representing the partitions. From these sections electrotypes were taken, and thus the veritable figures were used to illustrate the absolute structure of the comb. The results of these brilliant researches were published in the "Proceedings of the American Academy of Sciences."
[To be continued.]
- An address delivered at the meeting of the American Association for the Advancement of Science. Read at Buffalo, New York, August, 1876. By Edward S. Morse, Vice-President Biological Section.
- Gray's "Darwiniana," p. 284.
- Journal of the Boston Society of Natural History, vol. iv., p. 368.
- "Proceedings of the Philosophical Academy," vol. iii., p. 7.
- American Journal of Science and Arts, vol. vii., 1829.
- "Proceedings of the Boston Society of Natural History," vol. vii., p. 231, December 15, 1860.
- Ibid., vol. xv., p. 315.
- Ibid., vol. v., p. 121.
- "Proceedings of the Boston Society of Natural History," vol. v., pp. 121-128.
- Proceedings of Cincinnati Society of Natural Science, No. 1, June, 1876.
- "Proceedings of the American Academy," vol. v., p. 72.
- "Proceedings of the California Academy of Natural Science," vol. v., p. 128.
- American Journal of Science and Arts, vol. xli., January and March, 1866.
- "Proceedings of the Boston Society of Natural History," vol. xv., p. 156.
- American Journal of Science and Arts, vol, iv., December, 1872, p. 454, and vol. v., p. 39.
- "Proceedings of the Boston Society of Natural History," vol. xiv., p. 276.
- "Memoirs of the American Academy," vol. vi., pp. 377-458 (1859).
- "Proceedings of the Boston Society of Natural History," vol. ix., p. 230.
- "On Phytophagic Varieties and Phytophagic Species," "Proceedings of the Entomological Society of Philadelphia," vol. iii., p. 403.
- "Proceedings of the Boston Society of Natural History," vol. xiv., p. 141.
- Popular Science Monthly, vol. vi., p. 481.
- "Proceedings of the Boston Society of Natural History," vol. ix., p. 276.
- American Naturalist, vol. vi., p. 721.
- Ibid., vol. viii., p. 237.
- "The American Beaver and his Works."
- "Butterflies of North America," part ix.
- American Naturalist, vol. viii., p. 257.
- "Proceedings of the American Academy," vol. viii., p. 349.
- "Proceedings of the American Academy of Arts and Sciences," vol. xxii., p. 301.
- "Proceedings of the Buffalo Society of Natural Science," vol. i., p. 130.
- "Proceedings of the American Academy," vol. vii., p. 68.