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PALAEONTOLOGY.

a sudden alteration of the depth suited to its existence. When the change is only indepth, those species which have the power of existing through a great range will be,as it were, singled out of the mass, and continue to live, whilst others will dwindleaway and die; but when the change of depth is accompanied by a change of seabottom, the effect will be at once a diminution in the number of species, and also acessation of some of the families. Professor Forbes has also justly remarked that thelong-continued and undisturbed existence of Testacea, on any ground, must, by theaccumulation of their dead debris, render that ground unfit for the continuation oflife until it has been covered with a new layer of sedimentary matter, uncharged withorganic contents,—so that in this way beds rich in organic remains may alternatewith others containing none; a phenomenon well known to the Geologist. Everyspecies has, according to Professor Forbes, three maxima of development,—in depth,in geographic space, in time. In depth, it is at first represented by few individuals,which become more and more numerous, until it has attained that precise zonewhere all circumstances most favour its growth, when it begins to diminish in num-ber, and at length disappears. So, also, in geographical range, it multiplies until ithas attained the climate most suitable to its growth; and in time, or geologicalrange, a similar maximum may be observed; but in this case it should be premisedthat the duration of a species ought generally to fall within a geological formation.

In applying, therefore, these results of an inquiry into the fauna of the sea bottomto the investigation of more ancient faunse, the Geologist must bear in mind thatthere may have been then, as now, a beginning and minimum of development ofparticular species in some one section of a formation, an increase in its development,as it proceeded from the centre of creation, until it had attained its maximum, andthen a gradual diminution to a second minimum. The final limitation or destruction,by whatever cause it may have been produced, of the fauna of a formation, was nottherefore always coincident with the duration of a species, but often took place whenit was at its maximum of development, as is strikingly exhibited in the fauna of theChalk, and has been pointed out in the remarks upon the various genera formed afterthe type of Ammonites, and which, having attained a very high degree of development,were abruptly cut short, and ended with that formation. The study of the sea bot-tom, as manifested by the nature of the mineral deposits, and the classification of thegenera and species according to the probable depth of their usual habitat, mustnecessarily precede any deductions as to the climate and position of the deposit. Normust it be forgotten that the limits of the several zones of existence may in the fauna;of former epochs have been greatly modified by climate. There is, however, onefeature of extinct fauna; which deserves special notice, as being illustrated by thefauna; of deep zones,—namely, the abundance of Brachiopoda . Were, indeed, theevidence afforded by the genus Terebratula of our recent fauna considered conclusive,the greater proportion of geological formations must have been produced in deepwater; but here it may be wise to exercise some caution, and to consider that when-ever a class undergoes so great a development in families and genera, some of itsforms have been intended to take the place of the species of other classes, and thushave naturally a greater range.

It is thus that Palaeontology , viewed as a branch of Zoology , has not only broughtbefore the Geologist proofs of the existence of numerous forms of organic beings,which were associated together in groups not yet fitted for the reception of Man, andwhich have passed away with the creations of which they were a part,—but has alsoenabled him to reason with certainty, by reference to the natural characters andhabits of extinct animals and plants, on the varying mineral conditions of each suc-cessive stage in the Earth’s History. He is perhaps surprised at first to find how