48 NOTICES OF NEW BOOKS. 



of those species found in the neocomian (lower greensand), albian 

 (gault and upper greensand) and chalk groups are common to either 

 of the others. M. d'Orbigny goes yet further, and states that he 

 has not found any species common to the turonian and senonian 

 portions of the chalk*, or to the neocomian and aptian divisions 

 (the lower greensands and gault) of the neocomian group. These 

 results arrived at by M. d'Orbigny inspire great confidence ; and it is 

 left to future investigators first to demonstrate where the universality 

 of these differences stops, and then to modify from these considera- 

 tions the classification of the strata. 



" I must not quit the discussion of so important a law without one 

 more observation. I have said that the demonstration of the law 

 must be effected by a knowledge of facts ; but there occur, though 

 rarely, cases in which these facts may be interpreted differently, ac- 

 cording as the preconceived opinion of the palaeontologist who de- 

 scribes them is favourable or otherwise to the truth of the law. 

 Such cases may seem to serve for arguments on both sides ; and a 

 simple reference to some well-marked genus will exemplify the 

 meaning and truth of this. If, for instance, we compare the skele- 

 tons of existing species of hares, we can scarcely, with regard to 

 some of them, find any distinctive characters. If then a fossil hare 

 is discovered, especially if we only obtain a few fragments, it may 

 happen that these may seem identical with some of the recent spe- 

 cies. The palaeontologist who examines these remains may therefore 

 either declare that the species is identical with those now living, or 

 that it is an extinct species, of which the distinctive characters pro- 

 bably existed in the soft parts and were not sufficient to be indicated 

 in the skeleton. The rarity of such instances, and their small im- 

 portance for the determination of strata, prevent any real confusion 

 from being introduced by this cause." — Vol. i. pp. 58-66. 



Law 2. " The differences which exist between extinct and existing 

 groups of species are greater in proportion \as the faunas are more 

 ancient ; or, in other words, the more ancient the formation^ the 

 more widely do its fossil contents depart from the existing type.'' — 

 P. 67. 



The author states that this law is distinctly exemplified whenever 

 we compare the fossils of different periods ; and people are not un- 

 frequently inclined to designate new forms as strange and anomalous 

 when they differ widely from those with which we are familiar. 



It appears, if we attempt to express the amount of this difference, 

 that in the most recent beds the species are generally referable to 

 existing genera; that descending to those at somewhat greater 

 depths we must group them into new genera ; and that in the older 

 formations additional orders and families require to be introduced. 



' * [These two names have been assumed by some French geologists to designate 

 the lower and upper portions of the chalk ; the former including a portion of the 

 upper greensand, the craie tufau, the glauconie crayeuse, and the craie chloritee ; 

 and the latter the chalk with flints.— Ed.] 



