| OCR Text |
Show 186 DARWINIAN A. to frequent and great changes of habitation o.r.limitation, but without appreciable change of speCific form or character ·' that is ' without profounder change.s tha.n those within which a species at the present time IS known to vary. :Moreover, be is careful to state that he is far from concluding that the time of_ the appearance of a species in Europe at all indi~ates the tim~ of its origin. Looking back still further Into the Tertiary epoch, of which the vegetabl~ re~ains indicate many analog.ous, but few, jf 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, ftnally, that "the theory of. a successi~n of forms through the deviation of anterior forms IS t~e most natural hypothesis, and the most acco::dant With the known facts in palreontology, geographical botany and zoology, of anatomical structure and class~fication.: but direct proof of it is wanting, and moreover, It 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 hI.S t orw. epoch . " In contemplating the present state of the species of Oupuliferw in Europe, De Oandolle comes to the conclusion that, while the beech is increasing, and, extending its limits southward and westward (at the expense of Ooniferce and birches), the c~mmon oak, t? some extent' and the Turkey oak decldedly, are .d i- minishing and retreating, and this wholly irrespective of man's agency. This is inferred of the Turkey ?ak from the great O'aps found in its present geographical area which are 5 otherwise inexplicable, and which he reg;rds as plain· indications of a partial extinction. SPECIES AS TO VARIATION, ETC. 187 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 Qike the American species) greatly te~d to vary; that is, they manifest an active disposition to produce new forms. Every form tends to become hereditary, and so to pass from the state of mere variation to that of tace; and of these competing incipient races some only will survive. Quercus Robur offers a familiar illustration of the manner in which ·one form may in the course of time become separated into two or more distinct ones. To Linnreus 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. sessilijlora, and Q. pubescens. 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 Linnreus 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 co~clusioil be different, under a slight and very supposable change of circumstances. 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, sessiliflo'ra, and |
