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"bound to explain, or, at any rate, not to be inconsistent with, the "whole of the facts which it professes to account for; and if there is a "single one of these facts which can be shown to be inconsistent with "(I do not merely mean inexplicable by, but contrary to) the hypo"thesis, the hypothesis falls to the ground-it is worth nothing. "One fact with which it is positively inconsistent is worth as much, "and as powerful in negativing the hypothesis, as five hundred. If "I am right in thus defining the obligations of an hypothesis, "Mr. Darwin, in order to place his views beyond the reach of all "possible assault, ought to be able to demonstrate the possibility of "developing from a particular stock by selective breeding two forms, "which should either be unable to cross one with another, or whose "cross-bred offspring should be infertile with one another."

This has, however, been partially done by Mr. Darwin, in his remarkable researches on Dimorphism in certain plants, particularly of the genera Primula and Linum. In his paper on the existence of two forms, and on their reciprocal sexual relations, in several species of the genus Linum.* Mr. Darwin has shown that in certain species of this genus the individuals are of two kinds, which "differ a little in structure, but greatly in function. The "foliage, corolla, stamens and pollen (examined dry and distended "with water) are alike in both forms. The difference is con"fined to the pistil: in the one form, which I will call short style, the column formed by the united styles and the short "stigmas together is about half the length of the whole pistil in "the other and long-styled form. Considering the slight

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"and variable differences between the two forms of this Linum, it is "not surprising that they have been hitherto overlooked." And yet each form is almost unaffected by the action of its own pollen, and requires to be impregnated by that of the other form.

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"If we are

"to take," says Mr. Darwin, " fertility as the criterion of distinctness, "it is no exaggeration to say that the pollen of the long-styled Linum "grandiflorum (and conversely of the other form) has been differentiated, with respect to the stigmas of all the flowers of the same form, to a degree corresponding with that of distinct species of the same genus, and even of species of distinct genera." Some naturalists, no doubt, will meet this argument by the assumption that the genus Linum was created under these two different forms, and with

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* Journ. Proc. Linn. Soc. Bot. vii. p. 69. See also Nat. Hist. Rev. 1863, p. 476.

this remarkable physiological peculiarity. How far Professor Huxley will feel disposed to do so it is not for us to determine; but we have here, at any rate, within the limits of one species two forms, the individuals of which are either altogether, or almost unable to cross with one another.

However this may be, we cordially agree with Professor Huxley, "that Mr. Darwin's work is the greatest contribution which has been "made to biological science since the publication of the Règne "Animal' of Cuvier, and the History of Development' of Von "Baer. I believe that, if you strip it of its theoretical part, it still "remains one of the greatest encyclopædias of biological doctrine "that any one man ever brought forth; and I believe that if you "take it as the embodiment of an hypothesis, it is destined to be the "guide of biological and psychological speculation for the next three "or four generations."

VII.-RECENTLY PUBLISHED BRITISH FLORAS.

(1.) NORTH YORKSHIRE; STUDIES OF ITS BOTANY, GEOLOGY, CLIMATE, AND PHYSICAL GEOGRAPHY. By John Gilbert

Baker, 8vo. London: Longmans. 1863.

(2.) FLORA OF SURREY. By James Alexander Brewer, sm. 8vo. London: Van Voorst, 1863.

(3.) FLORA OF MARLBOROUGH.

By the Rev. T. A. Preston, sm.

8vo. London: Van Voorst, 1863.

:

MR. BAKER'S book is divided into three parts. The first part being devoted to Geology, Climatology, and Lithology; a chapter to each subject. The second to Physical Geography and Topography and the third to Botany, consisting principally of a list of the Flowering Plants, Ferns, and Mosses of North Yorkshire, with numerous special localities, as well as general indications of the horizontal and vertical range of the species. The Botany of the book, however, is by no means confined to this third part. It is a very strong feature nearly all through.

The first chapter of Part I. is based upon Professor Phillips' writings, and is purely Geological. It is accompanied by a neatly executed Geological map of the North Riding.

The second chapter commences rather abruptly, with an explanation of the "Regions" and " Zones" of altitude proposed by Mr.

Watson, in his "Cybele Britannica." In Mr. Watson's book, Britain is divided, by the limit imposed by climate to cultivation, into an upper "Arctic" and a lower "Agrarian" Region, each Region being subdivided into three zones, upper, middle, and lower. Climate, as affected by latitude, precludes the extension of the lowest zone of the Agrarian region so far north as Yorkshire, the lower levels of which county belong to the middle zone of this region. And, on the other hand, the highlands of North Yorkshire do not rise to a sufficient altitude to reach even the middle zone of the Arctic region, the upper verge of which corresponds to the upper limit of the Crossleaved Heath, so that the entire area of the North Riding is embraced by three of Mr. Watson's zones, viz. the Middle Agrarian, the Upper Agrarian, and the Lower Arctic Zones.

The account of these altitudinal zones is followed by some details referring to temperature in the shade and sun. Then follow the characteristics of the zones of altitude, with much interesting

matter.

By far the largest proportion of North Yorkshire plants are "dwellers in the plains," disappearing, one after the other, as we climb. There are some, however, which have their head-quarters in the mountains, and which disappear as we descend. The former Mr. Baker speaks of as "ascending," the latter as "descending," or "montane" species; the montane species form in North Yorkshire about one-twelfth of the whole Phanerogamia and Ferns.

Of native species of flowering Plants and Ferns, Mr. Baker says, that from the sea level to

100 yards of elevation, there occur 86

per cent.

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The higher the elevation, the greater, it is observed, is the restriction both of available area and of kind of situation. Lists of species are also given as they die out at successive ascending "leaps" of 100 yards.

With reference to the contrast of the ascending and descending sets of plants, which the author makes very prominent, it does not

appear to us, that he has sufficiently pointed out the true nature of the distinction between them. His readers, unaccustomed to such matters, may reasonably conclude that the distinction between ascending and descending plants is an absolute one, while, in reality, it is no such thing. All the North Riding descending plants are ascending in countries farther north; all the ascending plants may be descending if we travel farther south. This Mr. Baker of course knows perfectly well, and we think some explanation should accompany this part of his subject.

The chapter on Climatology concludes with an account of the periodic phenomena of vegetation; of the temperature of springs and of the sea; of the rainfall and humidity in the Riding; and of the winds. Should a new edition of this work be called for, we would recommend a rearrangement of the subjects treated of in this chapter. We have noticed its contents in the order in which we encountered them from the beginning, and it must be apparent that they are not arranged in the most orderly sequence. The chapter is accompanied by a map, on which is indicated, in colours, the respective areas occupied in the Riding by the three altitudinal zones to which we have referred.

Chapter III. on Lithology also has a map, which is intended to show the areas characterised by three principal lithological peculiarities. The chapter includes the following sections. 1. Dysgeogenous and eugeogenous strata. 2. Their distribution in North Yorkshire. 3. Their influence upon the configuration of the country; and (4), upon the topography of the vegetation.

Following M. Jules Thurmann, who published a well-known essay on the Phytostatics of the Jura in 1849, Mr. Baker adopts the hardness, power of absorption and retention of moisture, and degree of resistance to abrading influences offered by rocks, as the basis upon which depend the principal geognostic relations of plants. And there can be no doubt that, at least in temperate climates, the distribution of species is dependent to a considerable extent upon such physical characteristics, yet it would not have been out of place in this essay, had some reference been made to the views of those who hold that the chemical composition of rocks exerts even a more important influence than their mechanical characteristics, upon the distribution of plants. The relation in which plants stand to the rocks upon which they grow, has long occupied Mr. Baker's attention, and we have a right to look for the expression of his opinion

upon this question, in connection, for example, with the patent in stance of plants which are generally acknowledged to affect calcareous formations; a number of which plants occur in North Yorkshire.

Dysgeogenous, means "sparingly-detritus-yielding;" Eugeogenous, "freely-detritus-yielding," and Mr. Baker arranges the Geological formations of the North Riding under these heads thus: Dysgeogenous. Magnesian Limestone and Middle Oolite. Subdysgeogenous. Mountain Limestone and Basalt.

Eugeogenous. Millstone Grit, New Red Sandstone, Lias, Lower Oolite, and Kimmeridge Clay.

By far the larger part of the Riding is eugeogenous, from the wide area occupied by the Lower Oolite, New Red Sandstone, and Millstone Grit, and from the comparative humidity of soils overlying these rocks, the vegetation of North Yorkshire is "of a predominantly damp-loving stamp."

With regard to the classification of rocks under these two groups, it would seem to us that so far as such physical features as affect the plant are concerned, that the same result may issue from very opposite conditions. Thus the requirements of a dry, or "Xerophilous," species may be met either by the features attributed to Dysgeogenous rocks, of hardness, impermeability and great resistance to disintegration, or by the converse condition of friability, permeability and great facility of abrasion. The true distinction would appear to hinge simply upon the capacity of the rock, or of the soil resulting from its wear, for the retention of moisture, rather than upon the diagnostic characters assigned.

And admitting this, we have at once the important element of climate introduced as a factor, since the surface evaporation from humid areas immediately affects temperature, which, whatever the rocks may say, we all know, has a primary influence even upon the local distribution of plants. Mr. Baker does not omit to point out the difference, in the capacity for retaining water, between argillaccous and sandy soils, but it is not clear how far he attributes the diverse relations in which they stand to certain plants, to a direct, or an indirect, influence.

Chapter IV. commences part second of the book, and is entitled -Drainage Districts and Geographical Categories of Plants.

The Riding is divided into nine districts based upon the river drainage, which districts are separated from each other by rivers or

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