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Popular Science Monthly/Volume 71/August 1907/The Place of Linnaeus In the Unfolding of Science: His Views on the Class Mammalia

< Popular Science Monthly‎ | Volume 71‎ | August 1907



IN order rightly to appraise the value of Karl von Linné's contributions to biological science, it is necessary to bear in mind two very axiomatic facts. Our first axiom is that Linnæus became a point of departure in the history of modern biology only because he was in turn the product of the intersection of a great number of important causal series, which ramify and intertwine indefinitely and stretch back into the remote past of every aspect of life. The second axiom is that every new idea, or, for that matter, every new event, is the fertile hybrid from the fortuitous crossing of two or more specifically distinct old ideas, or events. And in order that we may discern a few of these fortuitous crossings and follow a little some of these interminable and intricate streams of cause and effect, it may not be inappropriate, in connection with the two-hundredth anniversary of the birth of Linnæus, to touch briefly upon a series of seven epochs of thought, from Aristotle to Darwin; and further to glance at the principles and facts upon which Linnæus based his two great contributions to the broader knowledge of the class of which man is the dominating member.

Not to go back indefinitely, we begin with the Aristotelian or initial analytic epoch of the fourth century b. c. Aristotle's theory of the genetic relationship of the chain of beings from polyp to man did not, of course, materially influence Linnæus. The idea of evolution which St. Thomas Aquinas, the "princeps scholasticorum" understood and developed, was not destined to come to its fruition through the schoolmen or even in Linnæus or Cuvier. But the true relation of Aristotle's thought to that of Ray and Linnæus may be exhibited in the following well-known citations from "The Parts of Animals."[2]

Some animals are viviparous, some oviparous, some vermiparous. The viviparous are such as man, and the horse, and all those animals which have hair; and of the aquatic animals, the whale kind, as the dolphin and
cartilaginous fishes [in reference to the viviparity of certain sharks]. (Book I., chap. V.) Of quadrupeds which have blood and are viviparous, some are (as to their extremities) many-cloven, as the hands and feet of man. For some are many-toed, as the lion, the dog, the panther; some are bifid, and have hoofs instead of nails, as the sheep, the goat, the elephant, the hippotamus; and some have undivided feet, as the solid hoofed animals, the horse and ass. The swine kind share both characters. An allusion to the "mule footed" swine, monstrosities in which the median digits are fused and terminate in a solid composite hoof. II., chap. V.)

Ray and later writers probably had this passage in mind when they used the descriptive terms "multifido," "bifido," "solidungula," "ungulata," "unguicnlata," "fissipedes." Here also attention is directed to the feet as exhibiting characteristic differences.

Animals have also great differences in the teeth both when compared with each other and with man. For all quadrupeds which have blood and are viviparous have teeth. And in the first place some are ambidenta[3] (having teeth in both jaws); and some are not so, wanting the front teeth in the upper jaw.[4] Some have neither front teeth nor horns, as the camel; some have tusks, as the boar; some have not. Some have serrated teeth,[5] as the lion, the panther, the dog; some have the teeth unvaried,[6] as the horse and the ox; for the animals which vary their cutting teeth have all serrated teeth. No animal has both tusks and horns; nor has any animal with serrated teeth either of those weapons. The greater part have the front teeth cutting, and those within broad (chap. II.).

This passage evidently directed the attention of later writers to the importance of the teeth as a means of distinguishing and hence of classifying mammals, and we shall see that Ray and, later, Linnaeus were quick to avail themselves of the suggestion.

Although Whewell[7] proves that Aristotle was quite unconscious of the classification that has been ascribed to him, he also admits that "Aristotle does show, as far as could be done at his time, a perception of the need of groups, and of names of groups, in the study of the animal kingdom; and thus may justly be held up as the great figure in the Prelude to the Formation of Systems which took place in more advanced scientific times."

Whewell quotes passages that show recognition of the lack of generic names to denominate natural groups. Aristotle says that "Of the class of viviparous quadrupeds there are many genera,[8] but these again are without names, except specific names, such as man. lion, stag, horse, dog, and the like. Yet there is a genus of animals that have manes, as the horse, the ass, the oreus, the ginnus, the innus, and the animal which in Syria is called heminus (mule). . . . Wherefore," he adds (that is, because we do not possess genera and generic names of this kind), "we must take the species separately and study the nature of each." "These passages," Whewell continues, "afford us sufficient ground for placing Aristotle at the head of those naturalists to whom the first views of the necessity of a zoological system are due '" (op cit., p. 352).

It is not necessary to dwell on the fact that from the time of Aristotle and his classical successors to the, close of the middle ages Europe thought itself too much preoccupied to pay particular attention to natural history; on the one hand with world-wide displacements and readjustments of peoples and of institutions, and on the other hand with the development of the great body of religious and metaphysical doctrines. Even the next epoch requiring our attention, the scholastic epoch in the history of science, so far as natural history is concerned, is perhaps rather a further interregnum than an epoch, rather an era or lapse of uneventful time, than a time of the slow ascension of some great illuminative idea. The anthropocentric idea dominated in natural history as the geocentric idea dominated in astronomy and hence a knowledge of the real or supposed properties of animals, and especially of plants, was chiefly cultivated in connection with alchemy, magic, materia medica, etc. The medieval imagination, full of mysticism, eager for the uncanny and fantastic and teeming with images of the ubiquitous devils, flourished on the marvelous tales of a "Sir John Mandeville," and peopled the earth with the monsters which so long survived and ramped in the Terræ Incognitæ of world maps. In the schools citations from authorities were accepted in lieu of proof, and the simple zoology of Aristotle and the scriptures was deeply covered by the accretions of learned exegesis.

However, it must be remembered that scholasticism had reached its prime as far back as the thirteenth century, in the system of the illustrious St. Thomas Aquinas, and that while the renaissance movement was discovering new worlds in all directions, scholasticism in general (but with some brilliant exceptions) had reached the phylo-gerontic stage, and was producing all sorts of bizarre specializations in terminology and in dialectic.

Nevertheless, it can not be doubted that the very excesses of scholasticism stimulated the reactive return to experience, which gave rise incidentally to biological science. Furthermore, the schoolmen perpetuated and aroused interest in Aristotle's analyses, and gave currency to many methods of analysis and description. Among these we may cite the dichotomous method of division, which is a forerunner of modern classifications, and the logical concepts of genus and species. Especially noteworthy was the expansion of classical Latin into a highly specialized language of philosophy and science.

We now come to the renaissance epoch. Biological science, and especially zoology, did not respond fully to the impulse of the renaissance movement until literature, politics, astronomy and geographical discovery had made the most signal advances. Hence in Aldrovandi (1522-1605) and Gesner (1516-1565) the superstitions and myths of the middle ages still linger, while the systematic work of future generations is initiated in the extensive, illustrated catalogues and descriptions of plants and animals. On the philosophical side of zoology, the Englishman Wotton, in his "De Differentiis Animalium" (Paris, 1552,) "rejected the legendary and fantastic accretions [of medieval zoology] and returned to Aristotle and the observation of nature" (Lankester)[9] One of the contemporaries of Gesner and Wotton was the founder of anatomy, Andreas Vesalius (1514—1564), who boldly broke with tradition and declared that the source of knowledge of the human body should be not Galen, but the human body itself.

Near the end of this period, the botanist, Cesalpino (Cæsalpinus), of Arezzo (1519-1778), himself a celebrated scholastic philosopher, but animated also by the new idea of direct observation, published his voluminous work "De Plantis" (1583), In this work the confused arrangements of plants of the earlier herbalists are replaced by an orderly classification suggested by the brigades of an army, and founded upon the number, the position and the figure of the reproductive parts. He divided plants into ten great classes, which were again subdivided; to these assemblages he gave monomial names in substantive form. Linnæus says of him that, "though the first in attempting to form natural orders he observed as many as the most successful later writers." (Whewell, op. cit., pp. 282-283.)

We may venture to suggest as a reason for this precocious development of a natural classification of plants the very multiplicity of kinds and the large herbaria and horticultural gardens in existence which necessitated some sort of orderly arrangement, and which would assist the eager student to recognize related series. We note in contrast the delayed progress of the classification of the mammals due to the comparative fewness of known forms, the greater complexity of organization and the difficulties of observation.

Among those who contributed the data for Linnæus's generalizations, no name is more important, at least in the history of vertebrate zoology, than that of John Bay. Accordingly, the fourth epoch under consideration may be termed the Raian epoch, and culminates with the publication in 1693 of Ray's "Synopsis Methodica Aninialium Quadrupedum et Serpentini Generis," which is one of the great landmarks in the history of classification. Ray's debt to the past is shown in the facts that his very lucid tabular analyses of the common structural features of animals are arranged dichotomously, that in each division and subdivision a single adjective or adjectival phrase indicates the most important common feature of the animals in question, and that these terms are, as we have seen, in many cases borrowed from Aristotle.

Ray, like Linnæus, gave more attention to plants than to animals, and depended upon his colleague, Willughby, for much of the data, especially in the fishes. And, like Linnæeus also, Ray had a superb gift of order and a philosophical mind that made him a worthy countryman and contemporary of Sir Isaac Newton.

In his tabular analysis Ray distinctly foreshadows Linnæus in the following points:

1. The higher vertebrates are contrasted with the fishes as breathing by lungs instead of gills.

2. The whales are classed with the viviparous animals and expressly removed from the fishes, from which they were further distinguished by the horizontality of the tail fin. This step, however, was felt to be so radical that Ray afterwards constructed a definition which included both whales and fishes.

3. As remarked by Gill, the terrestrial or quadruped mammals are bracketed with the aquatic as "Vivipara," and contrasted with the "Ovipara" or "Aves." "The Vivipara are exactly coextensive with Mammalia, but the word vivipara was used as an adjective and not as a noun. Linnæus did not catch up with this concept till 1758, when he advanced beyond it by recognizing the group as a class and giving it an apt name."[10]

4. The double ventricle is noted as characteristic of both Tivipara and Ovipara.

5. In order to associate the "Manati" and other amphibious mammals with their terrestrial congeners the term "hairy animals" is employed as more comprehensive than quadrupeds.

Ray further set the standard for Linnæus in his concise descriptions of European and foreign mammals, especially those described by travelers in America and in the east. Ray often used the term "species" merely as the equivalent of the middle English "spece," which survives in our word "spice," and meant "kind"; it was also equivalent to the logical "species (cf. the Greek εὶδς) of the schoolmen and is exemplified in Ray and Willughby's "Historia Piscium" in such phrases as "clarias niloticus Belonii mustelæ fluviatilis." "bagre piscis barbati acaculeati species." But Ray also used the term species in quite a Linnæan manner, as in the names Ovis laticauda, Ovis strepsiceros, Ovis domestica, etc. In form, at least, this foreshadows the binomial system of nomenclature and the recognition of the species in general as an objective reality and the unit of classification. But the form of Ray's specific definitions seems to imply that the term "species" in Ray's mind was often more a "differentia," or specific adjective modifying the generic concept, than a fully-developed substantive name, and Ray did not apparently realize the convenience of applying the binomial method of nomenclature universally. Even Linnæus at first introduced the specific, "trivial," or common, name, merely as a marginal index or symbol of the full specific phrase. Ray recognized the considerable variability of species but believed also in their separate creation and fixity. Ray frequently adverts to the internal characters of animals, and even a cursory examination of his book shows that, even by his time, a considerable number of observations on the soft parts of animals had already accumulated.

The work of Ray in botany and zoology fully prepared the way for Linnæus himself, whose epoch may be designated as the Linnæan or legislative epoch, because in his time methods of description, of classification and laws of nomenclature became fully established and settled. Linnæus inherited from Ray and from the scholastic system, the dogma of the separate creation and objective reality of species which became developed and strengthened in his hands as a result of his observations. His dictum was "species tot sunt diversæ quot diversæ formæ ab initio sunt creatæ." The resemblance between members of a single species were hence held to be due to descent from an original pair, and the mutual infertility of species to be the natural penalty of the effort to traverse the gaps established from the beginning.

And now that we have traced briefly the steps leading up to Linnæus, a few words more will summarize the relation of Linnæus to his successors. The sixth epoch in the history of zoology extends from the latter part of the eighteenth to the middle of the nineteenth century, and may be called the anatomical epoch, because, through the labors of Cuvier and his great English pupil and successor, Richard Owen, the taxonomic studies of the Linnæan school were supplemented by the establishment and great development of the sciences of comparative anatomy and paleontology. Cuvier's researches in these sciences further extended the dogma of the fixity of species, but Owen, through his broader knowledge, gradually gave up the idea and became an evolutionist, although not a selectionist. The seventh epoch, the Darwinian, in which happily we are living, has seen the overthrow of the traditional doctrine of the fixity of species, and has initiated the reexamination of all cosmical and terrestrial phenomena in the light of the doctrine of evolution.

Thus the great lawgiver of natural history is seen in his proper perspective, inheriting, on the one hand, the language and general methods of the past and the doctrine of special creation; inheriting, on the other hand, the new spirit and contributions of Vesalius, Cesalpino, Ray and many others, and building upon this the foundations of modern botany and zoology.


In attempting to appraise Linnæus's contributions to the broader knowledge of the class of mammals, we must bear in mind what Dr. J. A. Allen has well shown, namely, that Linnæus was primarily a botanist, that his interest in mammals was incidental, his opportunities for studying them very limited and his first-hand knowledge of extra-European mammals practically nil; and finally that several of his ordinal groupings of mammals (e. g., rhinoceros with the rodents) now appear highly unnatural and even ludicrous. But there are certain considerations which may prevent us from thinking any the less of Linnæus's judgment and genius on that account.

Although Linnæus may have known very little about extra-European mammals, he had, nevertheless, a fairly good conception of the essential features of mammals as a class, as shown by his definition in the tenth edition of the "Systema Naturæ" (1758). Here in concise phrase he states that mammals have a heart with two auricles and two ventricles, with hot red blood, that the lungs breathe rhythmically, that the jaws are slung as in other vertebrates, but "covered," i. e., with flesh, as opposed to the "naked" jaws of birds; that the penis is intromittent. the females viviparous, that they secrete and give milk, that the channels of perception are the tongue, nose, eyes, ears and the sense of touch: that the integument is provided with hairs, which are sparse in tropical and very few in aquatic mammals: that the body is supported on four feet, save in the aquatic forms, in which the hind limbs are said to be coalesced into a tail (the only erroneous idea in the whole definition).

Many of these characters had previously been noticed by Ray in his description of the hairy quadrupeds: and it is not impossible that Linnæus may have been assisted to the comprehension of the essential features of the mammals through his friendship with Bernard de Jussieu, who is said by Isidore Geoffroy St. Hilaire to have induced him to include the Cetaceans in the class Mammalia; and possibly he owed something to the researches of Klein and Brisson. But Linnaeus's own studies in medicine, in Holland, doubtless made him familiar with the anatomy of at least one mammal, man, and on his journeys through the north of Europe he must have observed many mammals at close range.

All this prepared him for the clear recognition and emphasis of two facts of far-reaching importance. It was evidently well known that the anatomy of mammals was similar in plan, if not in detail, to that of man, and we find Descartes, for example, in his "Discourse on Method" (Part V., 1637) advising those who wished to understand his theory of the action of the lungs and circulatory system, "to take the trouble of getting dissected in their presence the heart of some large animal possessed of lungs, for this is throughout sufficiently like the human" (ital. mihi). And it was further known that of all animals the monkeys are most nearly like man, both externally and internally. This was asserted by Aristotle and other classical authors but was fully demonstrated in a carefully prepared and illustrated work[11] on the anatomy and appearance of animals from the Jardin du Roi, by a committee of savants of the French Academy, appointed by the Grand Monarch.

That this work and these important facts came under the notice of Linnæus on the occasion of his visit to Paris in 1738 is not improbable. At any rate, Linnæus did not hesitate to follow the logical consequences of these facts, namely, that in a strictly zoological classification man would be grouped not only in the class mammalia, but even in the same ordinal division with the monkeys. Accordingly, in the tenth edition of the "Systema" the earlier name Anthropomorphæ is replaced hx Primates, and the genera Homo, Simia, Lemur, Vespertilio are grouped under that order. The Primates were thus regarded as the chiefs of the graded hierarchy of terrestrial beings, and consequently, as in nearly all subsequent schemes down to the Darwinian epoch, head the classified legions of creatures. This allocation of man to the order Primates was surely an instance of Linnæus's genius in surmising the true affinities of puzzling organisms, and led the way to the modern generalization that man is knit by ties of blood kinship to the Primates, and more remotely to the whole organic world.

The diagnostic definition given by Linnæus of the order Primates may be cited because it rests upon the princi]iles and theories which guided him in classification and which led to his most successful groupings, as well as to his serious blunders. This definition is as follows:

Inferior front teeth iv. parallel, laniariform [canine] teeth solitary [i. e., in a single pair].
Mammæ pectoral, one pair.
The anterior extremities are hands.
The arms are separated by clavicles, the gait usually on all fours ("incessu tetrapodo volgo").
They climb trees and pluck the fruits thereof.

That this definition was insufficient to exclude all extraneous genera from this really natural order is evident from the facts: (1) That under Lemur Linnæus included not only all the then known forms now recognized as the suborder Lemuroidea, but also the "Flying Lemur" Galeopithecus, which properly either forms an order by itself with no near affinities with the Primates, or is at most a suborder of the Cheiroptera. (2) The definition also included the bats (Vespertilio), an order more nearly related to the Insectivores than to the Primates. Many of the characters selected by Linnaeus for his ordinal diagnoses were of the "adaptive" or superficial kind which are now known to have been most easily modifiable by changes in the external or internal environment. The reason for this mistake (a mistake from which few naturalists were free even down to our own generation) was that Linnæus regarded the mode of sustenance of a group as one of its most deep-seated attributes, most surely indicative of more or less hidden affinities with other groups. For it is well known that Linnæus was constantly searching for natural groups, although he did not realize that the natural affinity of the members of the larger groups was due to descent from common ancestors, just as in the case of members of the same species. An example of his reliance upon sustenance is seen in his definition, in the tenth edition of the "Systema," of the order Feræ, the Carnivora of later authors. Here "sustenance by rapine, upon carcasses ravenously snatched" is evidently felt to be connected with "front teeth in both jaws: superior vi, all acute," with "laniariform teeth [canines] solitary," with "claws on the feet acute." But one of his dicta in botany was that a character of great systematic importance in one group may be very variable in another, consequently he did not mention sustenance under Bruta, but contented himself with the two characters "front teeth none either above or below" and "gait awkward (incessus ineptior)." As this order included the elephant, the manatee, the sloth, the great anteater and the scaly ant eater, it has been justly cited as a grossly unnatural assemblage; and the grouping accounted for by Linnæus's ignorance of the animals composing it. But I am more disposed to attribute it first to his habit of searching for hidden affinities below the most obvious external differences, as when he placed the seals in the order Feræ, joined the bats with the Primates, the horse and the hippopotamus, the rhinoceros with the Rodents, and the pig with the Insectivores (in the order Bestiæ). That Linnæus recognized that the ordinal classification of the mammals was a difficult problem is shown by the conspicuous changes (not always improvements in our eyes) and redistributions which he made between the first and "tenth" editions of the "Systema" and further by the fact that Erxleben who revised and extended the Systema (1777) abandoned the ordinal divisions entirely and merely listed the genera seriatim. The difficulty of the problem in indicated by the fact that Cuvier, with far better material and more extensive knowledge, was constantly deceived by "adaptive" (or homoplastic) resemblances, and that even Professor Cope, who wrote much on homoplastic and convergent evolution, was himself deceived by the similarities of structure in the Marsupial "mole," Notoryctes, and the Cape Golden mole (Chrysochloris), an undoubted Insectivore. Furthermore, that even the most "inexcusable" blunder of Linnæus, that of placing rhinoceros with the Rodents under the order "Glires," was due, not to carelessness, but to the fact that the Indian rhinoceros has a single pair of close-set cutting incisors in the upper jaw which oppose the elongate incisor-like appressed canines of the lower jaw, and thus show a superficial approach to the Rodent dentition. If Linnæus had known that Hyrax, which Pallas described as a Rodent ("Cavia"), had cheek teeth like those of Rhinoceros, he doubtless might have felicitated himself upon his supposed astuteness.

In brief, Linnæus (as fully shown by Whewell[12]) from his profound and wide botanical knowledge, was acquainted with many natural orders and strove constantly to recognize others; he knew that a character of great diagnostic and fundamental value in one order may be of slight value in another; he knew that even in a natural order some of the diagnostic and fundamental characters might be absent in certain members otherwise clearly allied to a given series. He knew that a natural series is natural because of the totality of its characters, that the "genus makes the character" and not vice versa—a hard doctrine to many of his contemporaries. And when he had arrived at a conception of any given natural order he selected certain characters as diagnostic but not necessarily universal, and constructed professedly artificial or only partially natural keys to his "natural" orders.

Thus we may believe that when Linnæus turned his attention to the classification of animals he followed the same principles. And in this application of the principles gained in one subject to the data of another, we have a good example of the felicitous union of specifically distinct ideas to produce a line of ideas that were new and very fertile.

  1. This article is here published by courtesy of the council of the New York Academy of Sciences. It is largely adapted from a forthcoming memoir by the writer on the "History of the Classification of the Mammalia," prepared under the direction of Professor Henry Fairfield Osborn.
  2. Quoted by W. Whewell, "History of the Inductive Sciences," 8vo, London, 1837, Vol. III., pp. 347.
  3. Αμφόδοντα
  4. Χανλιόδοντα
  5. Καρχαρόδοντα
  6. Άνεπάλλακτα
  7. Op. cit., III., p. 350.
  8. Ειδη
  9. Lankester, E. Ray, The History and Scope of Zoology, in "The Advancement of Science," London, 1890, p. 293.
  10. "The Story of a Word—Mammal," Popular Science Monthly, Vol. LXI., September, 1902. pp. 434-438.
  11. "Mémoires pour servir à l'Histoire naturelle des Animaux," à la Haye, 1715 (4to, 2 vols.), Redigées par Perrault et Dodart.
  12. Op. cit., pp. 319-325.