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Show 192 D A.R WINIAN A. continuation, through numerous geological, geographical, and more recently historical changes, of anterior veo-etations the actual distribution of plants is seen to be0 a conseq'u ence of preceding conditions ; and geologi-cal 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 Oandolle 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 th~ 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 di-rection 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 main-tained along with a conviction of great persistence of specific characters. This is the idea of the excellent Swiss vegetable palreontologist, Heer, who imagines a sudden change of specific type at certain periods, and pe~haps is that of Pictet. Falconer adheres to SPECIES AS TO VARIATION, ETC. 193 somewhat ~i~ilar views in his elaborate paper on elep~ants, livmg and fossil, in the Natural History R~v~ew for January last. Noting that "there is clear evide~ce of the true mammoth having existed in Amenca long after the period of the northern drift w. hen. the surface of the countr.y had settled d own' mto Its present form, and also in Europe so late as to have been a contemporary of the Irish elk and on th other hand that it existed in England s; far back a: before the deposition of the bowlder clay; also t.hat four well-defined species of fossil elephant are known t have. existed in Europe; that "a vast number of th~ remams of three of these species have been exhumed ove: a · large area in Europe ; and, even in the geological sense, an eno~mous 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 spe~ific characters in an elephant," he presents the questwn, "Do, then, the successive elephants occurring in these strata show any signs of a passage 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 p~rsistence and uni~ormity o£ the characters of the molar teeth in the earliest known mammoth and his most modern successor .... Assuming the observation to be correct what strong proof does it not afford of the persistenc~ and constancy, throughout vast intervals of time of the distinctive characters of those organs which' are mos~ concerned in the existence and habits of the spemes ? If we cast a glance back on the long vista |
