Part 9 (1/2)
2. Although we are not quite sure how Dr Falconer in tends to apply the law of phyllotaxis to ill.u.s.trate his idea, we fancy that a pertinent ill.u.s.tration may be drawn from it in this way. There are two species of phyllotaxis, perfectly distinct, and we suppose, not mathematically reducible the one to the other, viz.: (1.) That of alternate leaves, with its vane ties and (2.) That of verticillate leaves, of which opposite leaves present the simplest case That although generally constant a change from one variety of alternate phyllotaxis to an other should occur on the same axis, or on successive axes, is not surprising, the different sorts being terms of a regular series--although indeed we have not the least idea as to how the change from the one to the other comes to pa.s.s But it is interesting and in this connection perhaps instructive, to remark that while some dicotyledonous plants hold to the verticillate, i.e., opposite-leaved phyllotaxis throughout, a larger number--through the operation of some deep seated and innate principle which we cannot fathom--change abruptly into the other species at the second or third node, and change back again in the flower, or else effect a synthesis of the two species in a manner which is puzzling to understand. Here is a change from one fixed law to another, as unaccountable, if not as great, as from one specific form to another.
An elaborate paper on the vegetation of the Tertiary period in the southeast of France, by Count Gaston de Saporta, published in the Annales des Sciences Naturelles in 1862, vol. xvi., pp. 309-344--which we have not s.p.a.ce to a.n.a.lyze--is worthy of attention from the general inquirer, on account of its a.n.a.lysis of the Tertiary flora into its separate types, Cretaceous, Austral, Tropical, and Boreal, each of which has its separate and different history--and for the announcement that ”the hiatus, which, in the idea of most geologists, intervened between the close of the Cretaceous and the beginning of the Tertiary, appears to have had no existence, so far as concerns the vegetation; that in general it was not by means of a total overthrow, followed by a complete new emission of species, that the flora has been renewed at each successive period; and that while the plants of Southern Europe inherited from the Cretaceous period more or less rapidly disappeared, as also the austral forms, and later the tropical types (except the laurel, the myrtle, and the Chamaerops humilis), the boreal types, coming later, survived all the others, and now compose, either in Europe, or in the north of Asia, or in North America, the basis of the actual arborescent vegetation. Especially ”a very considerable number of forms nearly identical with tertiary forms now exist in America, where they have found, more easily than in our soil--less vast and less extended southward--refuge from ulterior revolutions,” The extinction of species is attributed to two kinds of causes; the one material or physical, whether slow or rapid; the other inherent in the nature of organic beings, incessant, but slow, in a manner latent, but somehow a.s.signing to the species, as to the individuals, a limited period of existence, and, in some equally mysterious but wholly natural way, connected with the development of organic types: ”By type meaning a collection of vegetable forms constructed upon the same plan of organization, of which they reproduce the essential lineaments with certain secondary modifications, and which appear to run back to a common point of departure.”
In this community of types, no less than in the community of certain existing species, Saporta recognizes a prolonged material union between North America and Europe in former times. Most naturalists and geologists reason in the same way--some more cautiously than others--yet perhaps most of them seem not to perceive how far such inferences imply the doctrine of the common origin of related species.
For obvious reasons such doctrines are likely to find more favor with botanists than with zoologists. But with both the advance in this direction is seen to have been rapid and great; yet to us not unexpected. We note, also, an evident disposition, notwithstanding some endeavors to the contrary, to allow derivative hypotheses to stand or fall upon their own merits--to have indeed upon philosophical grounds certain presumptions in their favor--and to be, perhaps, quite as capable of being turned to good account as to bad account in natural theology.[IV-3]
Among the leading naturalists, indeed, such views--taken in the widest sense--have one and, so far as we are now aware, only one thoroughgoing and thoroughly consistent opponent, viz., Mr. Aga.s.siz.
Most naturalists take into their very conception of a species, explicitly or by implication, the notion of a material connection resulting from the descent of the individuals composing it from a common stock, of local origin. Aga.s.siz wholly eliminates community of descent from his idea of species, and even conceives a species to have been as numerous in individuals and as wide-spread over s.p.a.ce, or as segregated in discontinuous s.p.a.ces, from the first as at the later period.
The station which it inhabits, therefore, is with other naturalists in no wise essential to the species, and may not have been the region of its origin. In Aga.s.siz's view the habitat is supposed to mark the origin, and to be a part of the character of the species. The habitat is not merely the place where it is, but a part of what it is.
Most naturalists recognize varieties of species; and many, like De Candolle, have come to conclude that varieties of the highest grade, or races, so far partake of the characteristics of species, and are so far governed by the same laws, that it is often very difficult to draw a clear and certain distinction between the two. Aga.s.siz will not allow that varieties or races exist in Nature, apart from man's agency.
Most naturalists believe that the origin of species is supernatural, their dispersion or particular geographical area, natural, and their extinction, when they disappear, also the result of physical causes. In the view of Aga.s.siz, if rightly understood, all three are equally independent of physical cause and effect, are equally supernatural.
In comparing preceding periods with the present and with each other, most naturalists and palaeontologists now appear to recognize a certain number of species as having survived from one epoch to the next, or even through more than one formation, especially from the Tertiary into the post-Tertiary period, and from that to the present age. Aga.s.siz is understood to believe in total extinctions and total new creations at each successive epoch, and even to recognize no existing species as ever contemporary with extinct ones, except in the case of recent exterminations.
These peculiar views if sustained will effectually dispose of every form of derivative hypothesis.
Returning for a moment to De Candolle's article, we are disposed to notice his criticism of Linnaeus's ”definition” of the term species (Philosophia Botanica, No. 157): ”Species tot numeramus quot diversae formae in principio sunt creatae”-- which he declares illogical, inapplicable, and the worst that has been propounded. ”So, to determine if a form is specific, it is necessary to go back to its origin which is impossible A definition by a character which can never be verified is no definition at all.”
Now as Linnaeus practically applied the idea of species with a sagacity which has never been surpa.s.sed and rarely equaled and indeed may be said to have fixed its received meaning in natural history, it may well be inferred that in the phrase above cited he did not so much undertake to frame a logical definition, as to set forth the idea which, in his opinion, lay at the foundation of species; on which basis A.L. Jussieu did construct a logical definition--”Nunc rectius definitur perennis individuorum similium successio continuata generatione renascentium.” The fundamental idea of species, we would still maintain, is that of a chain of which genetically-connected individuals are the links. That, in the practical recognition of species, the essential characteristic has to be inferred, is no great objection--the general fact that like engenders like being an induction from a vast number of instances, and the only a.s.sumption being that of the uniformity of Nature. The idea of gravitation, that of the atomic const.i.tution of matter, and the like, equally have to be verified inferentially. If we still hold to the idea of Linnaeus, and of Aga.s.siz, that existing species were created independently and essentially all at once at the beginning of the present era, we could not better the propositions of Linnaeus and of Jussieu. If; on the other hand, the time has come in which we may accept, with De Candolle, their successive origination, at the commencement of the present era or before, and even by derivation from other forms, then the ”in principio” of Linnaeus will refer to that time, whenever it was, and his proposition be as sound and wise as ever.
In his ”Geographie Botanique” (ii., 1068-1077) De Candolle discusses this subject at length, and in the same interest. Remarking that of the two great facts of species, viz., likeness among the individuals, and genealogical connection, zoologists have generally preferred the latter,[IV-4] while botanists have been divided in opinion, he p.r.o.nounces for the former as the essential thing, in the following argumentative statement:
”Quant a moi, j'ai ete conduit, dans ma definition de l'espece, a mettre decidement la ressemblance au-dessus de caracteres de succession. Ce n'est pas seulement a cause des circonstances propres au regne vegetal, dont je m'occupe exclusivement; ce n'est pas non plus afin de sortir ma definition des theories et de la rendre le plus possible utile aux naturalistes descripteurs et nomenclateurs, c'est aussi par un motif philosophique. En toute chose il faut aller au fond des questions, quand on le peut. Or, pourquoi la reproduction est-elle possible, habituelle, feconde indefiniment, entre des etres organises que nous dirons de la meme espece?
Parce qu'ils se ressemblent et uniquement a cause de cela. Lorsque deux especes ne peuvent, ou, s'il s'agit d'animaux superieurs, ne peuvent et ne veulent se croiser, c'est qu'elles sont tres differentes. Si l'on obtient des crois.e.m.e.nts, c'est que les individus sont a.n.a.logues; si ces crois.e.m.e.nts donnent des produits feconds, c'est que les individus etaient plus a.n.a.logues; si ces produits euxmemes sont feconds, c'est que la ressemblance etait plus grande; s'ils sont fecond habituellement et indefiniment, c'est que la ressemblance interieure et exterieure etait tres grande. Ainsi le degre de ressemblance est le fond; la reproduction en est seulement la manifestation et la mesure, et il est logique de placer la cause au-dessus de l'effet.”
We are not yet convinced. We still hold that genealogical connection, rather than mutual resemblance, is the fundamental thing--first on the ground of fact, and then from the philosophy of the case. Practically, no botanist can say what amount of dissimilarity is compatible with unity of species; in wild plants it is sometimes very great, in cultivated races often enormous. De Candolle himself informs us that the different variations which the same oak-tree exhibits arc significant indications of a disposition to set up separate varieties, which becoming hereditary may const.i.tute a race; he evidently looks upon the extreme forms, say of Quercus Robur, as having thus originated; and on this ground, inferred from transitional forms, and not from their mutual resemblance, he includes them in that species. This will be more apparent should the discovery of transitions, which he leads us to expect, hereafter cause the four provisional species which attend Q. Robur to be merged in that species. It may rightly be replied that this conclusion would be arrived at from the likeness step by step in the series of forms; but the cause of the likeness here is obvious. And this brings in our ”motif philosophique.”
Not to insist that the likeness is after all the variable, not the constant, element--to learn which is the essential thing, resemblance among individuals or their genetic connection--we have only to ask which can be the cause of the other.
In hermaphrodite plants (the normal case), and even as the question is ingeniously put by De Candolle in the above extract, the former surely cannot be the cause of the latter, though it may, in case of crossing, offer occasion. But, on the ground of the most fundamental of all things in the const.i.tution of plants and animals--the fact incapable of further a.n.a.lysis, that individuals reproduce their like, that characteristics are inheritable--the likeness is a direct natural consequence of the genetic succession; ”and it is logical to place the cause above the effect.”
We are equally disposed to combat a proposition of De Candolle's about genera, elaborately argued in the ”Geographie Botanique,” and incidentally reaffirmed in his present article, viz., that genera are more natural than species, and more correctly distinguished by people in general, as is shown by vernacular names. But we have no s.p.a.ce left in which to present some evidence to the contrary.
V
SEQUOIA AND ITS HISTORY
THE RELATIONS OF NORTH AMERICAN
TO NORTHEAST ASIAN AND TO
TERTIARY VEGETATION
(A Presidential Address to the American a.s.sociation for the Advancement of Science, at Dubuque, August, 1872)