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Show 242 DARWINIAN A. not been exposed to the contagion, and of which a considerable proportion, under the. common 1~":" of atavism comes to be very much In the condition of a people invaded for the :first ti~e ~y the disease. To these as we might expect, vaccmatwn would prove a less s~feguard than to their progenitors three or four generations before. . . Mr. Bentham is a veteran systematic botamst of the highest rank and widest knowledge. He bad not, so far as we know, touched upon questions of origination in the ante-Darwinian era. The dozen of presi. dential addresses delivered at anniversary meetings of the Linnean Society, from his assumption of the chair in the year 1862 down to the current year-eacli devoted to some topic of interest-and his recent" Memoir on Oompositre," summing up the general results of a revision of an order to which a full tenth of all higher plants belong, furnish apt examples both of cautious criticism, conditional assent (as becomes the inaugurator of the quantification of the predicate), and of fruitful application of the new views to various problems concerning the classification and geographical distribution of plants. In his hands the hypothesis is turned at once to practical use as an instrument of investigation, as a means of !nterrogating Nature. In the result, no doubt seems to be left upon the author's mind that the existing species of plants are ·the result of the differentiation of previous species, or at least that the derivative hypothesis is to be adopted as that which offers the most natural, if not the only, explanation of the problems _concerned. Similar conclusions reached in this country, from a study of the ATTITUDE OF WORKING NATURALISTS. 243 relations o~ its present flora with that which in earlier ages occupied t~e arctic zone, might also be referred to. (See precedmg article.) An excellent instance of the way in which the derivative hypothesis is practically applied in these da s by a zoo"" l og.i st , I.s b ef ore us in Prof. Flower's moyd-' ~st and admirable paper on the Ungulata, or hoofed animals, and their geological history. We refer to it here, not so much for the conclusions it reaches or s~g~ests, as to com~end the clearness and the impart~ ahty of the hand~mg, and the sobriety and moderation of the deductwns. Confining himself "within the region of the known, it is shown that at least in one ~roup ~f animals, the facts which we 'have as yet acqmred pomt to the former existence of various interme~ iate forms, so numerous that they go far to discredit the view of the sudden introduction of new species. . . . The modern forms are placed alonO' lines which converge toward a common centre." Th~ gaps between the eA.risting forms of the odd-toed group of ungulates ~of. which horses, rhinoceroses, and tapirs, are the prmCipal representatives) are mostly bridged o~er by palreontology, and somewhat the same may be said of the even-toed group, to which the ruminants . and the porcine genus belong. "Moreover, the lines of both groups to a certain extent approximate but · within the limits of our knowledge, they do not ~eet: · .. Was the order according to which the introduction of new forms seems to have taken place since the Eocene then entirely changed, or did it continue as far back as the period when these lines would have been gradually fused in a common centre~ " |
