Darwiniana; Essays and Reviews Pertaining to Darwinism Part 8

Meanwhile an inevitable and legitimate hypothesis is on trial--an hypothesis thus far not untenable--a trial just now very useful to science, and, we conclude, not harmful to religion, unless injudicious a.s.sailants temporarily make it so.

One good effect is already manifest; its enabling the advocates of the hypothesis of a multiplicity of human species to perceive the double insecurity of their ground. When the races of men are admitted to be of one species, the corollary, that they are of one origin, may be expected to follow. Those who allow them to be of one species must admit an actual diversification into strongly-marked and persistent varieties, and so admit the basis of fact upon which the Darwinian hypothesis is built; while those, on the other hand, who recognize several or numerous human species, will hardly be able to maintain that such species were primordial and supernatural in the ordinary sense of the word.

The English mind is p.r.o.ne to positivism and kindred forms of materialistic philosophy, and we must expect the derivative theory to be taken up in that interest. We have no predilection for that school, but the contrary. If we had, we might have looked complacently upon a line of criticism which would indirectly, but effectively, play into the hands of positivists and materialistic atheists generally. The wiser and stronger ground to take is, that the derivative hypothesis leaves the argument for design, and therefore for a designer, as valid as it ever was; that to do any work by an instrument must require, and therefore presuppose, the exertion rather of more than of less power than to do it directly; that whoever would be a consistent theist should believe that Design in the natural world is coextensive with Providence, and hold as firmly to the one as he does to the other, in spite of the wholly similar and apparently insuperable difficulties which the mind encounters whenever it endeavors to develop the idea into a system, either in the material and organic, or in the moral world. It is enough, in the way of obviating objections, to show that the philosophical difficulties of the one are the same, and only the same, as of the other. IV

SPECIES AS TO VARIATION,

GEOGRAPHICAL

DISTRIBUTION,

AND SUCCESSION

(American Journal of Science and Arts, May, 1863)

Etude sur l'Espece, a l'Occasion d'une Revision de la Famille des Cupuliferes, par M. ALPHONSE DE CANDOLLE.-- This is the t.i.tle of a paper by M. Alph. De Candolle, growing out of his study of the oaks. It was published in the November number of the Bibliotheque Universelle, and separately issued as a pamphlet. A less inspiring task could hardly be a.s.signed to a botanist than the systematic elaboration of the genus Quercus and its allies. The vast materials a.s.sembled under De Candolle's hands, while disheartening for their bulk, offered small hope of novelty. The subject was both extremely trite and extremely difficult. Happily it occurred to De Candolle that an interest might be imparted to an onerous undertaking, and a work of necessity be turned to good account for science, by studying the oaks in view of the question of species. What this term species means, or should mean, in natural history, what the limits of species, inter se or chronologically, or in geographical distribution, their modifications, actual or probable, their origin, and their destiny--these are questions which surge up from time to time; and now and then in the progress of science they come to a.s.sume a new and hopeful interest. Botany and zoology, geology, and what our author, feeling the want of a new term proposes to name epiontology, [IV-1] all lead up to and converge into this cla.s.s of questions, while recent theories shape and point the discussion So we look with eager interest to see what light the study of oaks by a very careful experienced and conservative botanist, particularly conversant with the geographical relations of plants may throw upon the subject.

The course of investigation in this instance does not differ from that ordinarily pursued by working botanists nor, in deed are the theoretical conclusions other than those to which a similar study of other orders might not have equally led. The oaks afford a very good occasion for the discussion of questions which press upon our attention, and perhaps they offer peculiarly good materials on account of the number of fossil species.

Preconceived notions about species being laid aside, the specimens in hand were distributed, according to their obvious resemblances, into groups of apparently identical or nearly identical forms, which were severally examined and compared. Where specimens were few, as from countries little explored, the work was easy, but the conclusions, as will be seen, of small value. The fewer the materials, the smaller the likelihood of forms intermediate between any two, and--what does not appear being treated upon the old law-maxim as non-existent--species are readily enough defined.

Where, however, specimens abound, as in the case of the oaks of Europe, of the Orient, and of the United States, of which the specimens amounted to hundreds, collected at different ages, in varied localities, by botanists of all sorts of views and predilections--here alone were data fit to draw useful conclusions from. Here, as De Candolle remarks, he had every advantage, being furnished with materials more complete than any one person could have procured from his own herborizations, more varied than if he had observed a hundred times over the same forms in the same district, and more impartial than if they had all been ama.s.sed by one person with his own ideas or predispositions. So that vast herbaria, into which contributions from every source have flowed for years, furnish the best possible data--at least are far better than any practicable amount of personal herborization--or the comparative study of related forms occurring over wide tracts of territory. But as the materials increase, so do the difficulties. Forms, which appeared totally distinct, approach or blend through intermediate gradations; characters, stable in a limited number of instances or in a limited district, prove unstable occasionally, or when observed over a wider area; and the practical question is forced upon the investigator, What here is probably fixed and specific, and what is variant, pertaining to individual, variety, or race?

In the examination of these rich materials, certain characters were found to vary upon the same branch, or upon the same tree, sometimes according to age or development, sometimes irrespective of such relations or of any a.s.signable reasons. Such characters, of course, are not specific, although many of them are such as would have been expected to be constant in the same species, and are such as generally enter into specific definitions.

Variations of this sort, De Candolle, with his usual painstaking, cla.s.sifies and tabulates, and even expresses numerically their frequency in certain species. The results are brought well to view in a systematic enumeration:

1. Of characters which frequently vary upon the same branch: over a dozen such are mentioned.

2. Of those which sometimes vary upon the same branch: a smaller number of these are mentioned.

3. Those so rare that they might be called monstrosities.

Then he enumerates characters, ten in number, which he has never found to vary on the same branch, and which, therefore, may better claim to be employed as specific. But, as among them he includes the duration of the leaves, the size of the cupule, and the form and size of its scales, which are by no means quite uniform in different trees of the same species, even these characters must be taken with allowance. In fact, having first brought together, as groups of the lowest order, those forms which varied upon the same stock, he next had to combine similarly various forms which, though not found a.s.sociated upon the same branch, were thoroughly blended by intermediate degrees:

"The lower groups (varieties or races) being thus const.i.tuted, I have given the rank of species to the groups next above these, which differ in other respects, i.e., either in characters which were not found united upon certain individuals, or in those which do not show transitions from one individual to another. For the oaks of regions sufficiently known, the species thus formed rest upon satisfactory bases, of which the proof can be furnished. It is quite otherwise with those which are represented in our herbaria by single or few specimens. These are provisional species--species which may hereafter fall to the rank of simple varieties. I have not been inclined to prejudge such questions; indeed, in this regard, I am not disposed to follow those authors whose tendency is, as they say, to reunite species. I never reunite them without proof in each particular case; while the botanists to whom I refer do so on the ground of a.n.a.logous variations or transitions occurring in the same genus or in the same family. For example resting on the fact that Quercus hex, Q. coccifera, Q. acutifolia, etc., have the leaves sometimes entire and sometimes toothed upon the same branch, or present transitions from one tree to another, I might readily have united my Q. Tlapuxahuensis to Q. Sartorii of Liebmann, since these two differ only in their entire or their toothed leaves. From the fact that the length of the peduncle varies in Q. Robur and many other oaks, I might have combined Q. Seemannii Liebm. with Q. salicifolia Nee. I have not admitted these inductions, but have demanded visible proof in each particular case. Many species are thus left as provisional; but, in proceeding thus, the progress of the science will be more regular, and the synonymy less dependent upon the caprice or the theoretical opinions of each author."

This is safe and to a certain degree judicious, no doubt, as respects published species. Once admitted, they may stand until they are put down by evidence, direct or circ.u.mstantial. Doubtless a species may rightfully be condemned on good circ.u.mstantial evidence. But what course does De Candolle pursue in the case--of every-day occurrence to most working botanists, having to elaborate collections from countries not so well explored as Europe--when the forms in question, or one of the two, are as yet unnamed?

Does he introduce as a new species every form which he cannot connect by ocular proof with a near relative, from which it differs only in particulars which he sees are inconstant in better known species of the same group? We suppose not. But, if he does, little improvement for the future upon the state of things revealed in the following quotation can be expected:

"In the actual state of our knowledge, after having seen nearly all the original specimens, and in some species as many as two hundred representatives from different localities, I estimate that, out of the three hundred species of Cupuliferae which will be enumerated in the Prodromus, two-thirds at least are provisional species. In general, when we consider what a mult.i.tude of species were described from a single specimen, or from the forms of a single locality, of a single country, or are badly described, it is difficult to believe that above one-third of the actual species in botanical works will remain unchanged."

Such being the results of the want of adequate knowledge, how is it likely to be when our knowledge is largely increased? The judgment of so practised a botanist as De Candolle is important in this regard, and it accords with that of other botanists of equal experience.

"They are mistaken," he pointedly a.s.serts, "who repeat that the greater part of our species are clearly limited, and that the doubtful species are in a feeble minority. This seemed to be true, so long as a genus was imperfectly known, and its species were founded upon few specimens, that is to say, were provisional. Just as we come to know them better, intermediate forms flow in, and doubts as to specific limits augment."

De Candolle insists, indeed, in this connection, that the higher the rank of the groups the more definite their limitation, or, in other terms, the fewer the ambiguous or doubtful forms, that genera are more strictly limited than species tribes than genera, orders than tribes, etc. We are not convinced of this Often where it has appeared to be so, advancing discovery has brought intermediate forms to light, perplexing to the systematist. "They are mistaken, we think more than one systematic botanist will say, "who repeat that the greater part of our natural orders and tribes are absolutely limited," however we may agree that we will limit them. Provisional genera we suppose are proportionally hardly less common than provisional species; and hundreds of genera are kept up on considerations of general propriety or general convenience, although well known to shade off into adjacent ones by complete gradations. Somewhat of this greater fixity of higher groups, therefore, is rather apparent than real. On the other hand, that varieties should be less definite than species, follows from the very terms employed. They are ranked as varieties, rather than species, just because of their less definiteness.

Singular as it may appear, we have heard it denied that spontaneous varieties occur. De Candolle makes the important announcement that, in the oak genus, the best known species are just those which present the greatest number of spontaneous varieties and sub-varieties. The maximum is found in Q. Robur, with twenty-eight varieties, all spontaneous. Of Q. Lusitanica eleven varieties are enumerated, of Q. Calliprinos ten, of Q. coccifera eight, * etc. And he significantly adds that "these very species which offer such numerous modifications are themselves ordinarily surrounded by other forms, provisionally called species, because of the absence of known transitions or variations, but to which some of these will probably have to be joined hereafter." The inference is natural, if not inevitable, that the difference between such species and such varieties is only one of degree, either as to amount of divergence, or of hereditary fixity, or as to the frequency or rarity at the present time of intermediate forms.

This brings us to the second section of De Candolle's article, in which he pa.s.ses on, from the observation of the present forms and affinities of cupuliferous plants, to the consideration of their probable history and origin. Suffice it to say, that he frankly accepts the inferences derived from the whole course of observation, and contemplates a probable historical connection between congeneric species. He accepts and, by various considerations drawn from the geographical distribution of European Cupuliferae, fortifies the conclusion--long ago arrived at by Edward Forbes--that the present species, and even some of their varieties, date back to about the close of the Tertiary epoch, since which time they have been subject to frequent and great changes of habitation or limitation, but without appreciable change of specific form or character; that is, without profounder changes than those within which a species at the present time is known to vary. Moreover, he is careful to state that he is far from concluding that the time of the appearance of a species in Europe at all indicates the time of its origin. Looking back still further into the Tertiary epoch, of which the vegetable remains indicate many a.n.a.logous, but few, if any, identical forms, he concludes, with Heer and others, that specific changes of form, as well as changes of station, are to be presumed; and, finally, that "the theory of a succession of forms through the deviation of anterior forms is the most natural hypothesis, and the most accordant with the known facts in palaeontology, geographical botany and zoology, of anatomical structure and cla.s.sification: but direct proof of it is wanting, and moreover, if true, it must have taken place very slowly; so slowly, indeed, that its effects are discernible only after a lapse of time far longer than our historic epoch."

In contemplating the present state of the species of Cupuliferae in Europe, De Candolle comes to the conclusion that, while the beech is increasing, and extending its limits southward and westward (at the expense of Coniferae and birches), the common oak, to some extent, and the Turkey oak decidedly, are diminis.h.i.+ng and retreating, and this wholly irrespective of man's agency. This is inferred of the Turkey oak from the great gaps found in its present geographical area, which are otherwise inexplicable, and which he regards as plain indications of a partial extinction. Community of descent of all the individuals of species is of course implied in these and all similar reasonings.

An obvious result of such partial extinction is clearly enough brought to view The European oaks (like the American species) greatly tend to vary that is they manifest an active disposition to produce new forms Every form tends to become hereditary and so to pa.s.s from the state of mere variation to that of race and of these competing incipient races some only will survive. Quercus Robur offers a familiar ill.u.s.tration of the manner in which one form may in the course of time become separated into two or more distinct ones.

To Linnaeus this common oak of Europe was all of one species. But of late years the greater number of European botanists have regarded it as including three species, Q. pedunculata, Q. sessiliflora, and Q. p.u.b.escens.

De Candolle looks with satisfaction to the independent conclusion which he reached from a long and patient study of the forms (and which Webb, Gay, Bentham, and others, had equally reached), that the view of Linnaeus was correct, inasmuch as it goes to show that the idea and the practical application of the term species have remained unchanged during the century which has elapsed since the publication of the "Species Plantarum." But, the idea remaining unchanged, the facts might appear under a different aspect, and the conclusion be different, under a slight and very supposable change of circ.u.mstances. Of the twenty-eight spontaneous varieties of Q.

Robur, which De Candolle recognizes, all but six, he remarks, fall naturally under the three sub-species, pedunculata, sessiliflora, and p.u.b.escens, and are therefore forms grouped around these as centres; and, moreover, the few connecting forms are by no means the most common. Were these to die out, it is clear that the three forms which have already been so frequently taken for species would be what the group of four or five provisionally admitted species which closely surround Q. Robur now are. The best example of such a case, as having in all probability occurred through geographical segregation and partial extinction, is that of the cedar, thus separated into the Deodar, the Lebanon, and the Atlantic cedars--a case admirably worked out by Dr. Hooker two or three years ago. [IV-2]

A special advantage of the Cupuliferae for determining the probable antiquity of existing species in Europe, De Candolle finds in the size and character of their fruits. However it may be with other plants (and he comes to the conclusion generally that marine currents and all other means of distant transport have played only a very small part in the actual dispersion of species), the transport of acorns and chestnuts by natural causes across an arm of the sea in a condition to germinate, and much more the spontaneous establishment of a forest of oaks or chestnuts in this way, De Candolle conceives to be fairly impossible in itself, and contrary to all experience. From such considerations, i.e., from the actual dispersion of the existing species (with occasional aid from post-tertiary deposits), it is thought to be shown that the princ.i.p.al Cupuliferae of the Old World attained their actual extension before the present separation of Sicily, Sardinia and Corsica, and of Britain, from the European Continent.

This view once adopted, and this course once entered upon, has to be pursued farther. Quercus Robur of Europe with its bevy of admitted derivatives, and its attending species only provisionally admitted to that rank, is very closely related to certain species of Eastern Asia, and of Oregon and California--so closely that "a view of the specimens by no means forbids the idea that they have all originated from Q. Robur, or have originated, with the latter, from one or more preceding forms so like the present ones that a naturalist could hardly know whether to call them species or varieties." Moreover, there are fossil leaves from diluvian deposits in Italy, figured by Gaudin, which are hardly distinguishable from those of Q. Robur on the one hand, and from those of Q. Douglasii, etc., of California, on the other. No such leaves are found in any tertiary deposit in Europe; but such are found of that age, it appears, in Northwest America, where their remote descendants still flourish. So that the probable genealogy of Q. Robur, traceable in Europe up to the commencement of the present epoch, looks eastward and far into the past on far-distant sh.o.r.es.

Quercus Ilex, the evergreen oak of Southern Europe and Northern Africa, reveals a similar archaeology; but its presence in Algeria leads De Candolle to regard it as a much more ancient denizen of Europe than Q.

Robur; and a Tertiary oak, Q. ilicoides, from a very old Miocene bed in Switzerland, is thought to be one of its ancestral forms. This high antiquity once established, it follows almost of course that the very nearly-related species in Central Asia, in j.a.pan, in California, and even our own live-oak with its Mexican relatives, may probably enough be regarded as early offshoots from the same stock with Q. hex.

In brief--not to continue these abstracts and remarks, and without reference to Darwin's particular theory (which De Candolle at the close very fairly considers)--if existing species, or many of them, are as ancient as they are now generally thought to be, and were subject to the physical and geographical changes (among them the coming and the going of the glacial epoch) which this antiquity implies; if in former times they were as liable to variation as they now are; and if the individuals of the same species may claim a common local origin, then we cannot wonder that "the theory of a succession of forms by deviations of anterior forms"

should be regarded as "the most natural hypothesis," nor at the general advance made toward its acceptance.

The question being, not, how plants and animals originated, but, how came the existing animals and plants to be just where they are and what they are, it is plain that naturalists interested in such inquiries are mostly looking for the answer in one direction. The general drift of opinion, or at least of expectation, is exemplified by this essay of De Candolle; and the set and force of the current are seen by noticing how it carries along naturalists of widely different views and prepossessions--some faster and farther than others--but all in one way. The tendency is, we may say, to extend the law of continuity, or something a.n.a.logous to it, from inorganic to organic Nature, and in the latter to connect the present with the past in some sort of material connection. The generalization may indeed be expressed so as not to a.s.sert that the connection is genetic, as in Mr.

Wallace's formula: "Every species has come into existence coincident both in time and s.p.a.ce with preexisting closely-allied species." Edward Forbes, who may be called the originator of this whole line of inquiry, long ago expressed a similar view. But the only material sequence we know, or can clearly conceive, in plants and animals, is that from parent to progeny; and, as De Candolle implies, the origin of species and that of races can hardly be much unlike, nor governed by other than the same laws, whatever these may be.

The progress of opinion upon this subject in one generation is not badly represented by that of De Candolle himself, who is by no means p.r.o.ne to adopt new views without much consideration. In an elementary treatise published in the year 1835, he adopted and, if we rightly remember, vigorously maintained, Schouw's idea of the double or multiple origin of species, at least of some species--a view which has been carried out to its ultimate development only perhaps by Aga.s.siz, in the denial of any necessary genetic connection among the individuals of the same species, or of any original localization more restricted than the area now occupied by the species. But in i855, in his "Geographic Botanique," the multiple hypothesis, although in principle not abandoned, loses its point, in view of the probable high antiquity of existing species. The actual vegetation of the world being now regarded as a continuation, through numerous geological, geographical, and more recently historical changes, of anterior vegetations, the actual distribution of plants is seen to be a consequence of preceding conditions; and geological considerations, and these alone, may be expected to explain all the facts--many of them so curious and extraordinary--of the actual geographical distribution of the species. In the present essay, not only the distribution but the origin of congeneric species is regarded as something derivative; whether derived by slow and very gradual changes in the course of ages, according to Darwin, or by a sudden, inexplicable change of their tertiary ancestors, as conceived by Heer, De Candolle hazards no opinion. It may, however, be inferred that he looks upon "natural selection" as a real, but insufficient cause; while some curious remarks upon the number of monstrosities annually produced, and the possibility of their enduring, may be regarded as favorable to Heer's view.

As an index to the progress of opinion in the direction referred to, it will be interesting to compare Sir Charles Lyell's well-known chapters of twenty or thirty years ago, in which the permanence of species was ably maintained, with his treatment of the same subject in a work just issued in England, which, however, has not yet reached us.

A belief of the derivation of species may be maintained along with a conviction of great persistence of specific characters. This is the idea of the excellent Swiss vegetable palaeontologist, Heer, who imagines a sudden change of specific type at certain periods, and perhaps is that of Pictet.

Falconer adheres to somewhat similar views in his elaborate paper on elephants, living and fossil, in the Natural History Review for January last. Noting that "there is clear evidence of the true mammoth having existed in America long after the period of the northern drift, when the surface of the country had settled down into its present form, and also in Europe so late as to have been a contemporary of the Irish elk, and on the other hand that it existed in England so far back as before the deposition of the bowlder clay; also that four well-defined species of fossil elephant are known to have existed in Europe; that "a vast number of the remains of three of these species have been exhumed over a large area in Europe; and, even in the geological sense, an enormous interval of time has elapsed between the formation of the most ancient and the most recent of these deposits, quite sufficient to test the persistence of specific characters in an elephant," he presents the question, "Do, then, the successive elephants occurring in these strata show any signs of a pa.s.sage from the older form into the newer?"

To which the reply is: "If there is one fact which is impressed on the conviction of the observer with more force than any other, it is the persistence and uniformity of the characters of the molar teeth in the earliest known mammoth and his most modern successor . . . a.s.suming the observation to be correct, what strong proof does it not afford of the persistence and constancy, throughout vast intervals of time, of the distinctive characters of those organs which arc most concerned in the existence and habits of the species? If we cast a glance back on the long vista of physical changes which our planet has undergone since the Neozoic epoch, we can nowhere detect signs of a revolution more sudden and p.r.o.nounced, or more important in its results, than the intercalation and sudden disappearance of the glacial period. Yet the 'dicyclotherian'

mammoth lived before it, and pa.s.sed through the ordeal of all the hard extremities it involved, bearing his organs of locomotion and digestion all but unchanged. Taking the group of four European fossil species above enumerated, do they show any signs in the successive deposits of a transition from the one form into the other? Here again the result of my observation, in so far as it has extended over the European area, is, that the specific characters of the molars are constant in each, within a moderate range of variation, and that we nowhere meet with intermediate forms." . . .

Dr. Falconer continues (page 80):

"The inferences which I draw from these facts are not opposed to one of the leading propositions of Darwin's theory. With him, I have no faith in the opinion that the mammoth and other extinct elephants made their appearance suddenly, after the type in which their fossil remains are presented to us.

The most rational view seems to be, that they are in some shape the modified descendants of earlier progenitors. But if the a.s.serted facts be correct, they seem clearly to indicate that the older elephants of Europe, such as E. meridionalis and E. antiguus, were not the stocks from which the later species, E. primigenius and E. Africa.n.u.s sprung, and that we must look elsewhere for their origin. The nearest affinity, and that a very close one, of the European E. meridionalis is with the Miocene E.

planifrons of India; and of E. primigenius, with the existing India species.

"Another reflection is equally strong in my mind--that the means which have been adduced to explain the origin of the species by 'natural selection,'

or a process of variation from external influences, are inadequate to account for the phenomena. The law of phyllotaxis, which governs the evolution of leaves around the axis of a plant, is as nearly constant in its manifestation as any of the physical laws connected with the material world. Each instance, however different from another, can be shown to be a term of some series of continued fractions. When this is coupled with the geometrical law governing the evolution of form, so manifest in some departments of the animal kingdom, e. g., the spiral sh.e.l.ls of the Mollusca, it is difficult to believe that there is not, in Nature, a deeper-seated and innate principle, to the operation of which natural selection is merely an adjunct. The whole range of the Mammalia, fossil and recent, cannot furnish a species which has had a wider geographical distribution, and pa.s.sed through a longer term of time, and through more extreme changes of climatal conditions, than the mammoth. If species are so unstable, and so susceptible of mutation through such influences, why does that extinct form stand out so signally a monument of stability? By his admirable researches and earnest writings, Darwin has, beyond all his contemporaries, given an impulse to the philosophical investigation of the most backward and obscure branch of the biological sciences of his day; he has laid the foundations of a great edifice; but he need not be surprised if, in the progress of erection, the superstructure is altered by his successors, like the Duomo of Milan from the Roman to a different style of architecture."

Entertaining ourselves the opinion that something more than natural selection is requisite to account for the orderly production and succession of species, we offer two incidental remarks upon the above extract.

1. We find in it--in the phrase "natural selection, or a process of variation from external influences"--an example of the very common confusion of two distinct things, viz., variation and natural selection.

The former has never yet been shown to have its cause in "external influences," nor to occur at random. As we have elsewhere insisted, if not inexplicable, it has never been explained; all we can yet say is, that plants and animals are p.r.o.ne to vary, and that some conditions favor variation. Perhaps in this Dr. Falconer may yet find what he seeks: for "it is difficult to believe that there is not in nature a deeper-seated and innate principle, to the operation of which natural selection is merely an adjunct." The latter, which is the ensemble of the external influences, including the compet.i.tion of the individuals them selves, picks out certain variations as they arise, but in no proper sense can be said to originate them