Popular Science Monthly/Volume 4/March 1874/Professor Louis Agassiz
|PROFESSOR LOUIS AGASSIZ.|
OF THE CAMBRIDGE MUSEUM OF COMPARATIVE ZOOLOGY.
LOUIS JEAN RODOLPHE AGASSIZ, whose death occurred the 14th of last December, was born May 28, 1807, in Mottier, Switzerland. From his earliest childhood he evinced a remarkable fondness for the study of natural science, and before he had left school began to collect and study into the habits of fishes. Having finished his course at the Gymnasium of Bienne, he chose for his profession that of medicine, and commenced to study at the Academy of Zurich. Thence he went to Heidelberg, where he made a special study of anatomy. He next entered the University of Munich, where, in company with Martius, Oken, Döllinger, and Schelling, he devoted himself eagerly to the pursuit of natural history. At that time Martius was publishing his great work on the "Natural History of Brazil," arid, upon the death of Spix, who was editing the zoological portion, Martius intrusted to Agassiz the description of the fishes. In this work, which was admirably well done, Agassiz characterized nine genera, embracing forty-two species new to science.
For some time Agassiz had contemplated a monograph on the "Fresh-water Fishes of Central Europe," but pecuniary embarrassment rendered this impossible, till a bookseller by the name of Cotta, to whom Agassiz showed the material he had collected, furnished him the means necessary for its completion. Meanwhile he studied and obtained the degree of Doctor of Medicine at Munich, and of Doctor of Philosophy at Erlangen. After his examination, Agassiz went to Vienna, and applied himself closely to the study of actual and fossil ichthyology. From Vienna he went to Paris, where he made the acquaintance of Cuvier and Humboldt, both of whom warmly welcomed this expert young naturalist. Here he lived on the most intimate terms with Cuvier till the death of that naturalist, in 1832, when he returned to Switzerland and established himself at Neufchatel, where he was appointed Professor of Natural History, a position he held till his departure for America.
Through the influence of Humboldt, between whom and Agassiz there existed the warmest friendship, he was enabled to begin the publication of his "Poissons Fossiles," a work evincing such careful and profound research, and such a wonderful power of generalization, as to obtain for him a place among the very first naturalists of the day. This work, which appeared in parts, between the years 1833 and 1845, comprises five volumes, of about 1,700 quarto pages, with an atlas of 400 folio plates, and contains descriptions of nearly a thousand species of fossil fishes. Aside from the great number of species, genera, and families established, Agassiz adopted an entirely new system of classification. In the classification proposed by Cuvier, fishes were divided into two orders, according to the nature of the skeleton, viz., cartilaginous and osseous. Agassiz—looking upon the external covering of the animal as a reflex of the connection existing between the being and its surroundings, bearing the imprint of all the peculiarities of its existence, and consequently of its organization—deemed that the true principle of the classification of fishes was to be found in the scales. In view of this he proposed a division of the families of fishes into four orders, viz., Placoids, in which the scales are represented by plates of enamel, as in the sharks; Ganoids, in which the scales consist of angular bony plates covered with a thick layer of enamel, as in the gar-pikes; Ctenoids, or fishes with true scales, in which the posterior edges of the laminæ are toothed; and Cycloids, in which the scales are composed of simple laming with smooth posterior edges. This last order Agassiz subdivided into Acanthopterygian and Malacopterygian Cycloids, or fishes having two dorsal fins, one spiny and the other soft, and those having one soft dorsal fin. Agassiz found that the study of fossil fishes exhibits a remarkable parallelism between the development of the individual and that of the class in geologic time. During part of the embryonic life of fishes, and even in some adult forms, the dorsal cord exists as a simple gelatinous cylinder, surrounded by a fibrous sheath, in which, after a time, there is found a cartilaginous and then an osseous deposit, which goes to form the vertebrae, the ossification taking place first in the apophyses. This embryonic character Agassiz found to be peculiar to the fossil fishes of the earlier geologic ages. There is no trace of a vertebra, but the apophyses, usually ossified, rest directly on the spinal cord.
Regarding the permanence of type, the author found the species of one formation specifically distinct from those of another, and, while it is impossible to say that the species pass from one into another, as they appear and disappear suddenly without direct connection with their predecessors, yet, as a whole, they present a continual progress of development, from the lowest to the highest, and demonstrate most palpably the existence of an ever-present directive intelligence.
Up to the end of the Jura epoch there exists among fishes a uniformity of type as well as a uniformity in the different parts of the animals themselves. The Placoids and Ganoids were the only fishes then inhabiting the seas; but, as we approach the Jurassic period which became preeminently the age of reptiles, we find a remarkable abundance of Sauroids, which, in their osteological character, the organization of their soft parts, and their dermal integuments, approach so nearly the reptile Saurians. At the end of the Jura period we find the Ganoids and Placoids giving way to the Ctenoids and Cycloids, which at present constitute the majority of our fishes. In the chalk-group, two-thirds of the species belong to extinct genera; in the inferior tertiary, one-third. In the Norfolk clay and Molasse formations the genera, for the most part, approach those of the tropical seas of the present day; and in the Geodian clay of Greenland there is found a species identical with one now living. In addition to the description of the species, which occupies the bulk of the work, a chapter is devoted to a critical review of the fishes of Monte Bolca, and another to those of collections in England and Scotland.
Agassiz next turned his attention to the study of Mollusca and Echinoderms, and in 1836 published a prodromus of the Echinoderms, and in 1837 a treatise on the fossil Echinoderms of Switzerland. In 1839 he began a more elaborate work, entitled "Monographies d'Échinoderms vivant et fossile," a most important contribution to modern zoology. This work comprises five parts: the first and second, on the Salenies and Scutellæ, by Agassiz; the third and fourth, on the Galerites and Disaster, by Desor; and the fifth, on the Anatomy of Echinus, by Valentin. Among the Scutellæ Agassiz found well-marked differences between the living and fossil species, and that all the species of the genera Mellita, Rotula, and Encope, belong to the existing epoch. He found, moreover, that the species increase in size as they approach the present period. While he was publishing his work on the Echinoderms, this indefatigable naturalist also described and figured a large collection of fossil shells from the Oölite and Cretaceous formations, in a work entitled "Études critique sur les Mollusques de Jura et de la Craie," besides an annotated German translation of Buckland's "Geology," and French and German translations of Sowerby's "Mineral Conchology."
Notwithstanding the immense amount of work on his hands, Agassiz found time to prosecute his investigation upon the fresh-water fishes of Europe. The first part of this work, issued in 1839, is devoted to the genera Salmo and Thymallus. It appears as a folio of twenty-seven excellent plates, with descriptions illustrating six species of Salmo and one of Thymallus, one plate of each species being colored according to life, the others representing differences of age, sex, and locality. The second part, which did not appear till 1842, consists of a folio of plates and a volume of text on the "Embryology of the Salmons," by Carl Vogt, whom Agassiz had associated with him in his work. M. Vogt has given the most detailed descriptions and figures of the different organs, and the changes they undergo from the formation of the cellules, out of which the organs are developed, to the adult state. In regard to cell-formation, M. Vogt differs from Schwann in affirming that the germinative vesicle is formed prior to the nuclei and nucleoli. The volume closes with a history of the daily development of the embryo, from the exclusion of the egg to the birth of the young. This excellent work was never completed; Agassiz's departure for the United States shortly after, and his increasing responsibilities, prevented the perfection of his orignal plan.
In 1842 Agassiz began the publication of his "Nomenclator Zoologicus," an alphabetical list of every genus, with the name of the author, the work in which it originally appeared, the derivation of the name, and the family to which the genus belongs; the list embracing upward of seventeen thousand names. In the introduction the author examines the rules proposed by the British Association and those of the British Committee. He agrees with the rules proposing that the name given by a founder of a group, or the first describer of a species, should be retained, and that priority is to be conceded only to a name published in some universally accessible work. On the other hand, he objects to the restriction of priority to Linnæus and to the rule that would change a name previously in use in connection with some other genus in zoology or botany, as this would result in the sacrifice of half the names of recent times. He does not think it wise to discard barbarous names, and, when a species becomes the type of a new genus, he would retain the former specific name as the generic appellative. He objects to the use of small initial letters for substantives borrowed from persons or places, to the uniform restriction of family and sub-family terminations to idœ and inœ, and strongly condemns the proposition that the name of the original propounder of a species should be retained when the species is transferred to a different genus. He likewise condemns those who would change the authority for a genus when the name is changed through faulty orthography, and censures the use of vernacular names in scientific works to the exclusion of the systematic ones.
The "Fossil Fishes" was now approaching completion, but, in consequence of additional material, Agassiz determined to publish a supplement; and accordingly there appeared, in 1844, the "Fossil Fishes of the Old Red Sandstone." It was accompanied by an atlas of thirty-nine folio plates illustrative of the seventy-six species described. The author, after discussing the relative rank of the members of the various classes of the animal kingdom, and showing how closely the time of their appearance on the earth corresponds to their relative standing in their respective classes, announces the conclusions to which a study of the fishes of the Devonian system had led him. These fishes actually represent the embryonic age of the Reign of Fishes, undergoing "phases of development analogous to those of the embryo, and similar to the gradations which the present creation shows us in the ascending series it presents when viewed as a whole." The members of the five families whose species he describes, are characterized by the absence of distinct vertebræ, the apophyses resting on the spinal cord, and by the absence of ossification in the internal case of the cranium. In these characters, as well as in the peculiar development of the vertical fins, the heterocercal tail, the flattened form of the head, and inferior or sub-inferior mouth, we see peculiarities of structure common to the embryo and the lower forms of existing as well as paleozoic fishes. This affords us a key to the relative rank of these fishes, for we find the Cephalaspides, which recede most from the existing forms, confined to the Devonian. The Sauroids, which are represented only by a particular group—the Dipterians—are likewise confined to the Devonian. The Acanthodians become extinct at the end of the Chalk, while the Cestraciontes persist to the present epoch. The same year Agassiz also read before the British Association a "Report on the Fossil Fishes of the London Clay."
Of all Agassiz's investigations, perhaps none made his name more popularly known than his studies on glaciers—studies which were pursued through a long course of years, and conducted with the same painstaking care that had heretofore characterized all his labors.
About the year 1834, M. Charpentier advanced the theory that the erratic blocks, and certain dikes of peculiar shape found in the Alpine regions, were the result of the action of former glaciers descending from the Alps and reaching even the upper portions of the Jura. This theory Agassiz deemed improbable, till, having visited Charpentier and investigated the phenomena, he not only became convinced of the correctness of Charpentier's views, but deduced from these and other phenomena a theory which, at the time (1837), was startlingly novel. It was that, previous to the elevation of the Alps, the globe experienced a very great reduction of temperature, and that the appearance of those mountains found the surface of the globe, from the north-pole to the Mediterranean Sea, covered with an immense sheet of ice. An elevation of temperature, consequent upon the upheaval of the Alps, caused this ice slowly to disappear, remaining longest in the valleys, where it gradually retreated to its present limits, leaving behind it, as a record, the peculiar phenomena which have attracted the attention of so many observers.
Of course a theory so novel at once raised a storm of opposition, and it became necessary for Agassiz, if he would prove the correctness of his views, to make the most careful and thorough investigations on living glaciers. For this purpose Agassiz, in company with Desor and several others, made visits in 1838 and 1839 to the glaciers of Mont Blanc and the Bernese Oberland, and in 1840 established himself for the summer on the glacier of the Aar. That year he published his "Études sur les Glaciers," giving the results of his investigations up to that time. He also visited England, Scotland, and Ireland, and studied the evidences of ice-action in those countries.
But his labors were not finished. Doubting the sufficiency of the theory of De Saussure—that the cause of the motion of the glacier depends upon gravity—and inclined to accept the dilatation theory of Schenchzer, it became necessary for him to examine with care the structure, form, distribution, and rate of motion of the glacier. Thus it was that, in 1841, he began a second series of observations for the purpose of determining these points. He chose, for the theatre of his investigations, the glacier of the Aar, which, by its extent and accessibility, promised the most favorable results. In 1845 he had completed his work, and in 1847 appeared his "Système Glaciaire," which embodied the final results of his researches upon the structure of glaciers, and their effects upon the soil. The results at which he arrived may be summarized as follows: The glacier is a mass of ice reclining on the side of a mountain-ridge, or inclosed in a mountain-valley; it is always descending, and, while wasting away from heat at its lower extremity, is continually augmented at its source. The primary material of glacier-ice is the snow which falls in the high regions of the mountain. The yearly addition of snow in the higher cold regions gradually forces the snow down the valley; here, subject to alternate thawing and freezing, it undergoes a second crystallization into what is called névé snow, and still farther down, under increased pressure, comes transformed into a granular, opaque, bulbous ice, called névé ice, which at last changes into the compact blue ice of the glacier proper.
During the summer, when the snow-fall is at its minimum, the surface of the snow in the high regions becomes covered with dirt and sand, which the next winter covers with another snow-sheet. In summer these layers of snow, from a partial melting and subsequent freezing, become changed to ice on their surfaces, so that we have three kinds of deposits—beds of snow, sheets of dust, and layers of ice. As the whole is pushed down into the valley, these layers tend to assume a vertical position from the bottom of the mass, moving faster than the top; and, the snow in summer melting from the surface as far as the snow-line, the edges of the layers are found passing transversely across the glacier. The middle of the glacier being deeper, moves faster than the sides, and, the lower layers advancing more rapidly than the upper ones, the strata become curved forward, the lower layers being more sharply arched. The arch thus becomes the measure of the rate of movement in the different parts of the glacier. From this it will be seen that Agassiz dissents from the theory of Tyndall, which represents the stratified lines as due to ice-cascades, or breakages of the glacier in passing over sharp angles.
All glaciers exhibit numerous blue bands, which are parallel to the planes of stratification, and are formed by thawing and freezing, and by the vertical pressure of snow in the névé. Moreover, there are found certain veined structures of the ice which appear to be bands of infiltration, and intersect the planes of stratification at every possible angle. As they are most numerous at the sides of the glacier, it is probable that Tyndall's theory of internal liquefaction of ice by pressure may account for them.
In the progress of the glacier, its rate of movement is not uniform, the differences between the centre and the sides being about as ten to one. Neither is the motion uniform along the axis; the advance being greatest about half-way down the region of the névé, and diminishing in rapidity both above and below. Agassiz found that it was from 20 to 50 feet per year in the higher portions, about 250 feet in the névé, and diminishing again lower down.
The causes of the movement of the glacier are several. The weight of the glacier alone is not sufficient to propel it, as in this case the greatest movement would be in the winter, which is not the case. The principal agent is the infiltration of water, which is greatest when the winter snows are melting. The granular snow of the névé incloses numerous particles of air, which, when the snow is compressed into ice, form a net-work of capillary fissures that serve as canals of infiltration, the water in which, freezing, aids in propelling the glacier. Added to this may be such other causes as the weight of the mass, the pressure of accumulated snow above, the weight of infiltrated water, and the softening of the ice by water, and a consequent sliding along the surface.
The sides as well as the bottom of the glacier are studded with bowlders, pebbles, and sand, forming a gigantic rasp. As the glacier moves forward, this rasp grinds, furrows, and polishes the rocks over which it moves, the furrows trending in the direction in which the glacier moves. These furrows and polished surfaces, which are often observed on rocks remote from any living glaciers, are the record of the former existence of glaciers in such places. When the ground is uneven, the eminences being small, and the hollows too deep and wide to be bridged over by the glacier, the ice-rasp rounds and polishes these knolls, forming those rounded elevations which have received the name of roches moutonnées. In consequence of the rocky walls above the sides of the glacier becoming warmed by the sun, the ice is melted near them, and hence the glacier becomes convex. Into these troughs the débris from the walls fall and form long lines of bowlders, pebbles, and sand, which are called lateral moraines. When two glaciers flow together, the two lateral moraines on the adjoining sides of each unite and form what is called a medial moraine. A third form, the terminal moraine, is the accumulation of sand and rocks which the glacier pushes before in its progress down the valley. In consequence of the increased rate of progression of the centre of the glacier, these terminal moraines assume a semicircular form, which, when the glacier retreats, consequent upon an excess of liquefaction over the snow-supply, leaves a crescentic wall across the valley, usually cut in two by the river flowing from the glacier. The erratic blocks which are found over most of the globe, accompanying scratched and polished rock-surfaces, are simply the bowlders of the surface of the glacier left on or near the spot where they stood when the glacier disappears.
In the fall of 1846 Agassiz sailed for the United States, on a mission from the Prussian Government. The warm reception which greeted him here, and especially the rare field for scientific research which this country afforded, determined him, the next year, to make America permanently his home. The professorship of Natural History in the Lawrence Scientific School at Harvard College being offered him that same year, he accepted it, and held the position till his death, with the exception of two years when he occupied the chair of Natural History in the University of South Carolina, at Charleston. In 1848, in connection with H. E. Strickland, he began the publication of a "Bibliographia Zoologiæ et Geologiæ." This work, which comprises a list of all the periodicals devoted to zoology and geology, and an alphabetical list of authors and their works in the same departments, was completed in four volumes, the fourth being published in 1854.
Agassiz's studies on the glaciers of Switzerland led him to expect to find in the United States many traces of former ice-action. Nor was he disappointed. He explored the country from the Atlantic to the Rocky Mountains, from the great lakes to the Gulf of Mexico, and everywhere, north of the thirty-fifth parallel of latitude, found evidence of glacial action, in erratic blocks, polished and striated rock-surfaces, and terminal moraines. Naturally this served to confirm him in his belief in the universality of the ice-period; and, upon his departure for Brazil, in 1865, he announced his confident expectation of finding records of the former existence of glaciers in that country; for, according to his belief, not only most of the Northern, but also most of the Southern Hemisphere was, during the glacial epoch, encased in ice. The evidences of glacial action in the United States are fully discussed by Agassiz in his "Lake Superior," a work on the physical character, vegetation, and animals, of Lake Superior, compared with those of other and similar regions.
Agassiz was a firm believer in the diversity of origin of the human race, and his views on this point are ably presented in the Christian Examiner for July, 1850, and in an introduction to Nott and Gliddon's "Types of Mankind." While denying the unity of origin of the races of mankind, he by no means denies their essential unity as one brotherhood. He regards all races of men as possessing in common the moral and intellectual attributes of humanity which raise them above the brutes. But intellectual relationship does not imply community of origin. The geographical distribution of animals shows that distinct zoological provinces are each characterized by peculiar fauna, and that therefore animals did not originate from a common centre nor from a single pair. The races of men, in their natural distribution, cover the same ground as the zoological provinces, and he believes there is every reason to suppose that these races originally appeared as nations in the regions they now occupy. That the differences at present observed between various races are primitive, and have not been the result of modification from one common ancestral type, he believed is evidenced by the monuments of Egypt, which show that, for 5,000 years, there has been no physical change in the negro and Caucasian.
In 1850 there appeared in the Transactions of the American Academy of Arts and Sciences an article on the naked-eyed Medusæ, being Part I. of Agassiz's "Contributions to the Natural History of the Acalephæ of North America." He includes all the naked-eyed Medusæ in one family, and shows that the number of tentacles and the position of the ovaries alone cannot be considered as family characters. The true characters consist in a gelatinous disk, with a reentering margin, along which passes a submarginal tube connecting with the circulatory tubes proceeding from a central digestive cavity. Upon the margin are the tentacles and eye-specks. The reproductive organs are situated along the circulatory tubes. The generation is alternate, one form being polyp-like, the other medusoid. The nervous cord follows the circular submarginal tube, and consists of several rows of nucleated cells, alternating one with another. It passes into the bulbs at the base of the marginal tentacles, in which are situated the eye-specks. A branch of nerve-thread passes also along each of the radiating circulatory tubes to the digestive cavity. The circulatory fluid is chyme, and not chyle, as it is in the Articulata and Mollusca. The author describes four species, and distinguishes a new genus—Nemopsis.
Ever since his arrival in America, Agassiz had been collecting material for a series of "Contributions to the Natural History of the United States." In 1857 appeared two volumes of these contributions; the first containing an "Essay on Classification" and the history of the North American Testudinata; the second on "the Embryology of the Turtle."
In the essay on Classification, Agassiz affirms that Nature is but the expression of the thought of the Creator, and that a true classification will be found to be but an unfolding of the plan of creation, as expressed in living realities; that these realities do not exist in consequence of the continued agency of physical causes, but appear successively by the immediate intervention of the Creator. We find in Nature a progressive series, from lower to higher forms; but it is not a uniform progress for the animal kingdom as a whole; neither is it a linear progress for the branches or classes, but a progress in which each type has usually been introduced by the creation of species belonging to one of its higher groups, for the earliest representatives of a class do not always seem to be the lowest. Yet, notwithstanding these downward steps, the progress has continually tended toward the production of higher and higher types, culminating at last in Man.
We find a parallelism between the geological succession of animals and the embryonic growth of their living representatives, as well as a parallelism between the geological succession of animals and their relative rank. The earlier types of animals were synthetic or prophetic, foreshadowing a future group. Types likewise culminated and disappeared in past ages—a feature parallel with the fact that in embryological development parts fulfill their end and then disappear.
Regarding Nature as the embodiment of a certain divine plan, Agassiz sought for a classification that should be the expression of this plan. Accordingly he based his classification on the following divisions, which he deemed covered all the categories of relationship, as far as their structure is concerned: Branches, or types characterized by their plan of structure; of these, he admitted four, Vertebrates, Articulates, Molluscs, and Radiates. Classes, which are characterized by the mode of execution of the plan. Orders, by the degree of complication of structure. Families, by the form as determined by structure. Genera, by the details of execution; and Species, by the relation of the individuals to one another, and to the world in which they live.
In Part II., Agassiz believes that the Testudinata constitute an order among the class of reptiles; that their essential character lies not so much in the shield as in the special development of different regions of the body, thus giving them the highest rank in the class. The shield is partly a portion of the skeleton, and partly an ossification of the skin or of the walls of the body.
The second volume treats entirely of the embryology of the turtle, and is illustrated with thirty-four plates. The egg originates from between the cells of the stroma, and is in itself the animal in the first stages of development. The eggs are laid once a year, and grow a long time before they are fecundated. From the first copulation to the laying, four years elapse, during which time eight copulations take place. The segmentation of the yolk takes place during the passage of the egg through the oviduct. From the segmentation of the yolk to the period of hatching, the egg passes through thirty-one stages of development.
The third volume of the "Contributions" appeared in 1860, and was devoted to the class of Acalephæ, the author treating specially of the order Ctenophoræ'. The Ctenophoræ Agassiz divided into three suborders: the Eurystomæ', embracing three families, the Saccatæ, with three families, and the Lobatæ, with five families. The fourth volume of the "Contributions," in 1862, concluded the Acalephæ, treating of the Discophoræ, Hydroidæ, and the Homologies of the Radiata.
Of the more recent labors of Agassiz in connection with the Museum of Comparative Zoology at Cambridge—of which he was the director, and to which, in later years, he devoted his whole attention—it is not necessary to speak. With his journey to Brazil in 1865, and his later expedition from Boston to San Francisco on the United States Coast Survey steamer Hassler in 1872, the public is already sufficiently familiar. His last days were devoted to the cause of education, in the establishment of a school of natural history at Penikese Island.
Of the man himself but a word is necessary. As a naturalist, Prof. Agassiz was unwearied in his devotion to his favorite pursuits. He worked early and late, often denying to himself the most necessary rest and recreation; and his remarkably strong constitution sustained him under a strain that would quickly have proved fatal to a man of less vigor. His mind was preëminently great; gifted with a wonderfully retentive memory, he combined with it a power of generalization and quick perception that places him next to Cuvier, whose disciple he was, and whom he seemed to imitate. In his methods of investigation he was perfectly honest, and, though many might differ from him in his conclusions, none could deny the absolute integrity of his convictions. In his intercourse with his fellow-men he was extremely affable and genial, and especially so toward the young. With inexperience he was most patient and painstaking, never wearying in his efforts to aid. Tolerant of ignorance where associated with modesty, he had but little patience with arrogance and ignorance combined. His students will all bear witness to the unvarying cheerfulness and ready sympathy in him they had learned to look up to as their master.