Popular Science Monthly/Volume 79/December 1911/Buffon and the Problem of Species II
|BUFFON AND THE PROBLEM OF SPECIES|
THE JOHNS HOPKINS UNIVERSITY
WE have thus far noted three generally disregarded but fundamental facts concerning Buffon's opinions about the nature of species. The first fact is that in his preliminary discourse in the first volume of the "Histoire Naturelle," in which he sought to apply the Leibnitian principle of continuity to natural history, Buffon's emphasis upon the continuity of the gradations between species probably had no evolutionary implications. The second fact is that the principal doctrine of this discourse is to the effect that only individuals exist in nature, while species exist only by grace of the human imagination, which, aided by human ignorance, sees sharp lines of cleavage among organisms where no such lines are. The third fact is that this doctrine was already tacitly but decisively abandoned in Buffon's second volume, where he represents species as real and well-marked natural entities, their limits being determined by the test of the sterility of the products of cross-breeding. There are, indeed, many later passages where the old phraseology incongruously recurs; but it recurs in contexts in which the reality of species is expressly insisted upon.
2. When the fourth volume of the "Histoire Naturelle"—the first dealing specifically with the lower animals—appeared in 1753, four years after the first three, Buffon's departure from the notions set forth in the preliminary discourse became still more evident. He had by this time, in the first place, been greatly impressed by the homologies in the structure of the vertebrates; he had come to see some significance in those facts of comparative anatomy which his own treatise—though more through the contributions of Daubenton than through his own—was for the first time setting in a clear light. The existence throughout at least all the immensely diverse vertebrate forms of an underlying unity of type, Buffon was, I suppose, the first to bring forcibly to the attention of naturalists and philosophers, as a fact calling for serious consideration and explanation.
But consideration of the anatomical homologies did not lead Buffon merely to pious reflections. He saw clearly and unequivocally declared that this unity of type forcibly suggests the hypothesis of community of descent. To one who considers only this class of facts, he wrote:
Buffon thus presented the hypothesis of evolution with entire definiteness, and indicated the homological evidence in its favor. But did he himself regard that evidence as conclusive, and therefore accept the hypothesis? The passage cited is immediately followed by a repudiation, ostensibly on theological grounds, of the ideas which he has been so temptingly presenting.
This repudiation has been regarded as ironical, or as inserted merely pro forma, by those interpreters of Buffon who have made him out a thorough-going evolutionist. Unfortunately, nearly all these writers—dealing somewhat less than fairly with their readers—have failed to mention that his rejection of the evolutionary hypothesis was not put forth by him as resting exclusively upon these religious considerations. If the words just quoted stood alone, it would, indeed, be scarcely possible to take them seriously. But they do not stand alone; they are directly followed by arguments of quite another order against the possibility of the descent of one real species from another; and the essence of the most emphasized of these arguments lies in the Buffonian conception of the nature of species, already expounded in the second volume. In other words, the fact of the sterility of hybrids, and certain other purely factual considerations, were urged by Buffon as conclusive objections against the theory of descent.
Specifically, his arguments against evolution are three: (1) Within recorded history no new true species (in his own sense of the term) have been known to appear. (2) There is one entirely definite and constant line of demarcation between species: it is that indicated by the infertility of hybrids.
But why, it may be asked, should the sterility of hybrids be a proof of the wholly separate descent of the two species engendering such hybrids? This question Buffon does not neglect to answer. An "immense and perhaps an infinite number of combinations" would need to be assumed before one could conceive that "two animals, male and female, had not only so far departed from their original type as to belong no longer to the same species—that is to say, to be no longer able to reproduce by mating with those animals which they formerly resembled—but had also both diverged to exactly the same degree, and to just that degree necessary to make it possible for them to produce only by mating with one another." The logic of this is to me, I confess, a trifle obscure; but it is evident that Buffon conceived that the evolution from a given species of a new species infertile with the first could come about only through a highly improbable conjunction of circumstances. (3) Buffon's third reason for maintaining the fixity of species is the argument from the "missing links."
Taking these three arguments into account, then, Buffon arrives at this conclusion:
However plausibly Buffon's incidental expressions of deference to the testimony of revelation may be regarded as perfunctory and insincere, it would be absurd to suppose that he was also ironical in these legitimate and ostensibly scientific (however poor) arguments for the fixity of species—arguments which are closely connected with that conception of the nature of species which was perhaps his most influential personal contribution to the biological ideas of his time. We must conclude, then, that, while he clearly envisaged the hypothesis of evolution as early as 1753, and recognized that there was some probable evidence in its favor, he then seriously believed that the preponderance of probability was enormously against it. It is certain that contemporary readers must have understood this to be his position.
The same doctrine—that true species, as determined by the sterility of hybrids, are real natural entities and constant units amid the otherwise infinitely variable phenomena of organic nature—is repeated and emphasized many times in subsequent volumes of the "Histoire Naturelle." Thus in volume five (1755) Buffon—trying to retain as much of the principle of continuity as could be made consistent with his present view—writes as follows:
In the same volume he insists upon the equal antiquity of all real species, in the very passage in which he emphasizes the possibility of a wide range of variation within the species:
This is advanced as a partial explanation of the extreme diversity of breeds in the canine species: the dog is a short-lived animal and has therefore been capable of a relatively great degree of diversification.
A little later (in Vol. VI., 1756) Buffon declares that "nature imprints upon every species its inalterable characters." In 1765—that is, at precisely the period during which we are told that Buffon "was expressing very radical views on the mutability of species "—we find him (in his "Second View of Nature," Vol. XIII.) giving his most extreme expression to the doctrine of the reality and constancy of genuine species. Here the language of the preliminary discourse concerning the relative significance in nature of the species and the individual has come to be completely reversed.
This sort of rhetoric is not the dialect of an evolutionist; it is almost that of a Platonist. And there is more in plainer language to the same effect:
Many years later still, in 1778, there appeared the sub-division of the "Histoire Naturelle" which Buffon's contemporaries regarded as his most brilliant and most significant work—the "Époques de la Nature." This was a resumption on a grander scale, and upon new principles, of the task attempted in the "Theory of the Earth" in the first volume, thirty years before—an outline of planetary evolution. To the diffusion of evolutionary ways of thinking in the larger and vaguer sense, this treatise was a contribution of capital importance. Into the details of Buffon's geology I do not wish to enter in this paper. But it is worth while for our purpose to recall one or two striking facts about the "Époques." In it the writer, whom a recent German historian of biology has declared to have had a too little developed sense for the historical or genetic aspect of nature, attempted, in a far more comprehensive, more definite and more impressive way than any of his predecessors, to write the history of the gradual development of our planet from the time when, an incandescent ball, it was separated from the sun. The task was, of course, undertaken prematurely; but Buffon not only made the need of its eventual achievement evident, but also indicated two of the essential means by which it was to be accomplished: the study of present phenomena which can throw light upon the past processes through which existing conditions have been brought about; and the study of those natural "monuments which we ought to regard as witnesses testifying to us concerning the earlier ages." He insisted, moreover, with the utmost plainness upon (as it was then regarded) the extreme antiquity not only of the earth, but also of organic life. And in doing so he showed himself not at all disposed any longer to permit "revelation" to settle scientific questions. "How," he writes, "some one will ask me, do you reconcile this vast antiquity which you ascribe to matter with the sacred traditions, which give to the world only some six to eight thousand years? However strong be your proofs, however evident your facts, are not those reported in the holy book more certain still? "Buffon replies that he has all possible respect for scripture, but that it always pains him to see it used in this way. Doubtless there is no real conflict between its testimony and that of science; and he thereupon introduces what I suppose is the first of the long series of reconciliations of Genesis and geology. The six days were not really days, but long periods of time, and so forth. But in any case, he concludes, the Bible was originally addressed to ignorant men at an early stage of civilization, and was adapted to their needs and their intelligence. Its science was the science of the time, and ought not to be taken too literally. Finally, it is to be noted that in the "Époques" Buffon ceased to talk of the simultaneous creation of all species, and advanced the doctrine of the gradual appearance of different sorts of animals in conformity with geological conditions.
If, then, Buffon was desirous of inculcating the theory of the mutability of species, here was the place in which, above all others, he might be expected to do so fully and unequivocally. But here once more we find him reiterating the substance of his old doctrine:
By the "larger species" here, Buffon means those of greater size, such as the elephant and hippopotamus; and when he says that these are "especially" invariable, he means, as the whole context shows, not that any other species ever departs from its specific type, but that in these larger creatures even the "accessory touches" have been comparatively little altered.
Thus, in a long series of passages, from 1753 on, we find Buffon reiterating with explicitness and emphasis the same teaching, which has, for him, its principal bases in two of his most cherished conceptions: namely, in his conviction that the sterility of hybrids shows that species are real "entities of nature"; and in his embryological theory of "organic molecules" and of the "internal mold" which "casts into its own shape those substances upon which it feeds" and "can operate in the individual only in accordance with the form of each species." One of the first of modern naturalists to make the idea of organic evolution familiar to his contemporaries and to discuss it seriously, Buffon repeatedly rejected that theory, at all periods of his career; and he did so, not from timidity merely nor from an affectation of deference to scriptural authority, but upon reasoned grounds which he plainly stated and had every appearance of presenting as conclusive. Yet it is also undeniable, as will presently be seen, that he did not maintain this negative position without occasional waverings and doubts and at least one clear, though possibly inadvertent, self-contradiction.
3. In spite of his habitual emphasis upon the constancy of true species, Buffon insisted more than any of his predecessors, and more, perhaps, than any of his contemporaries, except Maupertuis and Diderot, upon the variability of organisms and the potency of the forces making for their modification.
The passage is from one of Buffon's later writings; but its close counterpart is to be found as early as 1756:
For the most part these changes were clearly represented by Buffon as taking place only within the limits of species; they amounted merely to the formation of new "races" or varieties. Since his criterion of identity of species (the possibility of interbreeding) did not essentially depend upon morphological similarity, he could with consistency suppose the descendants of a given pair to have departed to a very great (though not to an indefinite) degree, in the course of ages, from the form and external characters of their ancestors. It was, in other words, characteristic of his biological system that he set up an absolute distinction between species and varieties, gave an extreme extension to the notion of a variety, and sought to reduce the number of separate species as much as possible, by assuming—until the establishment of the sterility of the hybrids should prove the contrary—that most of the Linnæan species were merely varieties descended from a relatively small number of original specific types. Near the close of his essay "De la dégénération des animaux" (1766), Buffon writes:
Here, clearly, is an evolutionary program, strictly limited by the assumption that there are irreducible ultimate species, yet tolerably ambitious: to regard all known kinds of quadrupeds as derived from thirty-eight original types, by modification in the course of natural descent; and to determine the general causes and conditions of the production of species in the ordinary sense, i. e., of relatively stable varieties. These ideas occurred to Buffon too late to be made use of in his general plan for the classification of the quadrupeds; that plan, it will be remembered, was formed while he was unluckily under the influence of the principle of continuity. But in the volumes on birds, of which the first appeared in 1770, he had the opportunity for a fresh start; and he took advantage of it to introduce a method of distinguishing and classifying species which—within the limits already indicated—is expressly evolutionary in its principles.
Even the groupings which he gives, Buffon adds, can not be regarded, in the existing state of knowledge, as correctly and exclusively enumerating all the apparent species which are really akin to one another. The number of separate species which he lists, he intimates, is probably much too great. But at all events, he concludes with pride, his work is the first real attempt at an ornithologie historique.
The purpose of the present inquiry does not call for any extended exposition of Buffon's views about the causes of modification in animals and the ways in which quasi-species are formed. In the essay "De la dégénération des animaux" the subject is discussed at the length of over sixty quarto pages; the theories there advanced have been sufficiently accurately summarized by many previous writers. In brief, the factors in modification which he mentions as the most important are changes of climate (in which the most potent influence is temperature), changes of food, and the effects of domestication. But it is evident that he also believed in a general tendency to variation in the germ, and in the influence of acquired habits, of the use and disuse of parts, and of acquired lesions and mutilations. Thus he explains the humps, and the callosities on the knees and chest, of the camel and the llama as due to the habits of those animals under domestication. Similarly, the callosities on the haunches of the baboons arise from the fact that "the ordinary position of these animals is a sitting one—so that the hard skin under the haunches has even become inseparable from the bone against which it is continually pressed by the weight of the body." These theories, of course, take for granted the inheritance of acquired characters, which Buffon also (less cautious here than Maupertuis) explicitly asserts. It is, I suppose, also well known that Buffon called attention (as Linnæus did independently) to the struggle for existence between species, due to the excessive multiplication of individuals, and pointed out how an equilibrium comes to be established (so long as external conditions remain constant) by means of this opposition.
Buffon, in fact, rather over-worked this notion of a stable equilibrium, which rested upon the assumption of an approximate equality among species in their endowment for the struggle for survival. This is perhaps one reason why it did not occur to him to think of that struggle as causing a process of natural selection, or to see in it a factor in the formation of so-called species.
4. It must be evident to the reader from all that precedes that Buffon's mind, throughout nearly the whole of his life, was played upon by two opposing forces. Quite apart from any illegitimate and external influences, such as fear of the ecclesiastics—of which too much has been made—his thought was affected by two conflicting sets of considerations of a factual and logical sort. He saw certain definite reasons for regarding species as the fundamental constants of organic nature; what those reasons were has been sufficiently indicated. But he also saw that there was some force in the argument from the homologies; and—what in his case was still more important—he was committed to the program of explaining the diversities of organisms, so far as might be, by the hypothesis of modification in the course of descent; he was deeply impressed by the fact of variability; and he held to a theory of heredity (namely, of the heritability of acquired characters) which acted as a sort of powerful undertow towards a generalized evolutionism. Add to this that he was little careful of consistency and extremely careful of rhetorical effect—and it is not surprising that he occasionally forgot one side of his doctrine in emphasizing the other. There is one and, so far as I can discover, only one passage in which he seems categorically to contradict his ordinary teaching of the impossibility of the descent of really distinct species, sterile inter se, from common ancestors. This occurs at the end of the chapter on "Animals Common to Both Continents" (Vol. IX., 1761).
Here Buffon seems either to have forgotten or to have deliberately discarded his own usual criterion of diversity of species. He does not propose to inquire whether the American species are capable of having fertile progeny when mated with their respective congeners in the old world, but predicates difference of species solely on the ground of dissimilarity of form; and to the distinct species so determined he attributes an identical origin. But it is possible that he has here merely lapsed (as he apparently does occasionally elsewhere) into the terminology in which he was brought up, and is using the word species in the Linnæan sense rather than in his own.
More significant, perhaps, than this possibly inadvertent inconsistency is the fact that, in his fourteenth volume (1766), Buffon seems to raise explicitly the question—though only as a question—whether, after all, descent with modification may not extend to species as well as varieties.
Even here one can not be wholly sure that Buffon is not referring to Linnæan species, and using the word genera to indicate what he usually means by species in the strict sense. Assuming, however, that he is speaking of "true" species, it must be observed that while he raises the question of their mutability, he does not answer it finally in the affirmative. For the passage is shortly followed by that cited earlier in this paper, in which Buffon, though he derives many species traditionally regarded as distinct from a common stock, yet finds even in "the first ages of nature" thirty-eight irreducible diversities of specific type among quadrupeds.
There is, however, one peculiarly interesting essay in which Buffon shows himself a little dubious even about that "most fixed point in nature" upon which his usual doctrine of the reality and constancy of species was based—namely, the fact of the sterility of hybrids. As I have already mentioned, this seemed to him so central a point in natural history that he for many years assiduously collected data concerning it, and caused experiments bearing thereon to be made and carefully recorded at his own estate at Montbard. The results of these inquiries, which he reports in the chapter "On Mules" (in the third supplementary volume, 1776), led him to the conclusion that hybrids are not necessarily without hope of posterity. On the testimony of an affidavit from a gentleman in San Domingo, Buffon declares that in hot climates mules have been known to beget offspring of mares, and females of their kind to breed with horses. "One was therefore wrong formerly in maintaining that mules are absolutely infertile." Other experiments in the crossing of goats and sheep, dogs and wolves, canaries and goldfinches, are recited; they all go to show that sterility is merely a question of degree.
All hybrids (mulets), says prejudice, are vitiated animals which can not produce offspring. No animal, say reason and experience, is absolutely infertile, even though its parents were of separate species. On the contrary, all are capable of reproduction, and the only difference is a difference of more or less.
That hybrids are relatively infertile, and probably incapable of breeding with one another, Buffon still maintains; "their infecundity, without being absolute, may still be regarded as a positive fact." Something, therefore, is still left of his test of unity of species. But now that it seemed to be reduced to a mere difference in degree, it was no longer the sharp-cut, decisive, impressive thing that it had at first appeared. And, feeling that his criterion of species had a good deal weakened, Buffon was led—not, indeed, even now to an altogether unequivocal affirmation of the descent of real species from one another—but to a confused, half-agnostic utterance, in which he seems to take at least the possibility of such descent for granted:
This passage certainly indicates a strong inclination towards an acceptance of a thorough-going doctrine of descent; yet in Butler's lengthy compilation of the evidences of Buffon's evolutionism it is not cited at all! The volume containing it, says Butler, offers "little which throws additional light upon Buffon's opinion concerning the mutability of species"! In truth, it offers one of the best of the extremely few passages which give some plausibility at least to the theory that Buffon was continuously working towards an unqualified transformism and actually arrived at that position in his later life. But if he reached it (which his language just quoted does not quite justify us in declaring) he did so only in a transient mood. For, as we have already seen, in 1779, in the "Époques de la Nature," we once more find him asserting—though no longer upon the ground of the sterility of hybrids—that the "constitutive form" of each separate species is the same to-day as in "the earliest ages."
5. It is more important, and it is commonly easier, to determine what opinions a man's writings tended to encourage than to determine what opinions he actually held. Mind-reading is perhaps no essential part of the history of science. If, then, in conclusion, we raise the more important question with respect to Buffon, it is evident that his work both fostered and hindered the propagation of evolutionary ideas in biology. Earlier than any other except Maupertuis, he put the hypothesis of organic evolution before his contemporaries in a clear and definite form. He called to their attention, also, the facts of comparative anatomy which constitute one of the principal evidences for that hypothesis. Throughout the rest of the century we never cease to hear about the wonderful "unity of type" characteristic of the vertebrates and perhaps of all living things. It was this consideration which led Kant as near to evolutionism as he ever came; Herder and Goethe are full of it, though the former never admitted its full evolutionary consequences; and all, it is evident, learned it directly from Buffon. He, says Goethe, was the first to recognize eine ursprüngliche und allgemeine Vorzeichnung der Tiere. Buffon, moreover, once and for all inscribed upon the program of natural history, as its primary problems, the reduction of the number of separate species to a minimum, the derivation of highly divergent forms from a common origin through natural descent, and the discovery of the causes and methods of modification. He, finally, did more than any one else to habituate the mind of his time to a vastly (though not yet sufficiently) enlarged time-scale in connection with the history of organic nature, a necessary prerequisite to the establishment of transformism.
These were great steps in the progress of evolutionism. But it is equally true that Buffon probably did more than any other eighteenth century writer to check the progress of evolutionism. He did so partly by the authority which, for his contemporaries, attached to those personal utterances of his favorable to the doctrine of immutability. These utterances were far more numerous and more categorical than those which could be quoted on the other side; and they certainly were not taken as ironical by the average reader of the period. But, what is still more important, Buffon put into currency what passed for a scientific and serious argument against any wholesale theory of descent. In the eyes of many learned men of his own and later generations, perhaps his chief single contribution to science was his definition of species. This, as I have recently pointed out, was regarded as of immense importance by Kant, and was, indeed, the starting point and the controlling principle of that philosopher's biological speculations. "It is Buffon," wrote Flourens as late as 1844," who has given us the positive character of a species." Now before the Buffonian criterion of species was propounded, there already existed a tendency towards evolutionism, fostered by the principle of continuity and by such embryological conceptions as those of Maupertuis—a tendency to disregard species altogether and to infer from the variability of individuals to an unlimited and rather promiscuous mutability of the successive generations of living things. If it had not been for Buffon, transformism would probably have developed at first through the increase and diffusion of this tendency; and its development might well, in that case, have been more rapid. But when species came to be regarded as real "entities of nature," determined by the objective criterion of the sterility of hybrids, this somewhat too facile evolutionism received a check, and a certain presumption of the constancy of true species seemed to be created. This presumption had all the more force because it left room for a large measure of mutability in the case of varieties, and thus gave a sort of appeasement to the strong impulse towards genetic modes of thought which was already active in the mid-eighteenth century. But more than all this, Buffon, as we have seen, from the first managed to associate with his definition of species the assumption that the sterility of one kind of animal when crossed with another was a character that (unlike almost all others) could not have been produced in the course of descent with modification. And this supposition that the sterility of hybrids was incapable of an evolutionary explanation long remained a serious obstacle to the acceptance of the theory of descent, even with those little influenced by theological prejudices against the theory. We find even Huxley in 1862 troubled over the difficulty. In his Edinburgh lectures of that year "he warned his hearers of the one missing link in the chain of evidence—the fact that selective breeding has not yet produced species sterile to one another." The doctrine of descent was merely to be "adopted as a working hypothesis, . . . subject to the production of proof that physiological species may be produced by selective breeding." Since Buffon appears to have been the first to emphasize the notion of physiological species, and to give currency to the supposition that the sterility of hybrids affords a presumption against any thorough-going transformism, he must be regarded as having done more than almost any man of his time to counteract the tendency which he also, perhaps, did more than any other to promote.
- "Hist. Nat.," IV., 1753, p. 383.
- These, the most definite and decisive words on the subject to be found anywhere in Buffon 's writings, have been strangely disregarded by most of those who have discussed his attitude towards evolutionism. Samuel Butler can scarcely be acquitted of suppressing the passage, fatal to his theory. For he quotes in full the opening part of the passage, leaving off abruptly at the point where Buffon begins to introduce his serious objections to the theory of descent. Cf. "Evolution Old and New," p. 91.
- P. 59 (italics mine).
- P. 194.
- P. 55.
- Vol. XIII., p. i.
- Vol. XIII., pp. vii, ix.
- "Hist. Nat.," Supp., V., p. 27.
- Supp., V., 1778, p. 3.
- Vol. VI., pp. 59-60. I have borrowed Butler's excellent rendering of this passage.
- Vol. XIV., 1766, p. 358.
- "Hist, des Oiseaux," I., 1770, preface.
- Cf. Lovejoy, "Some Eighteenth Century Evolutionists," Pop. Sci. Monthly, July, 1904, p. 248 n.
- "Hist. Nat.," V., 1755, p. 252.
- Just a year earlier we have found Buffon using the most exaggerated language possible about the changelessness of species.
- Vol. XIV., p. 335; italics mine.
- Supp., III., p. 20; the italics are Buffon's.
- Supp., III., 1776, pp. 32-33.
- "Evolution Old and New," p. 165.
- Popular Science Monthly, January, 1911, pp. 37-38.
- Cf. Lovejoy, Popular Science Monthly, July, 1904, p. 248.
- Huxley's "Life and Letters," I., 193.