PAL^OOTOLOGICAL COLLECTIONS SOI 



ous to state that our evidence is fragmentary in an extreme 

 degree. For instance, until recently not a land-shell was 

 known belonging to either of these vast periods, with the 

 exception of one species discovered by Sir C. Lyell and Dr. 

 Dawson in the carboniferous strata of North America ; but 

 now land-shells have been found in the lias. In regard to 

 mammiferous remains, a glance at the historical table pub- 

 lished in Lycll's Manual will bring home the truth, how 

 accidental and rare is their preservation, far better than 

 pages of detail. Nor is their rarity surprising, when we 

 remember how large a proportion of the bones of tertiary 

 mammals have been discovered either in caves or in lacus- 

 trine deposit):', ; and that not a cave or true lacustrine bed 

 is known belonging to the age of our secondary or palaeozoic 

 formations. 



But the imperfection in the geological record largely re- 

 sults from another and more important cause than any of 

 the foregoing ; namely, from the several formations being 

 separated from each other by wide intervals of time. This 

 doctrine has been emphatically admitted by many geologists 

 and palaeontologists, who, like E. Forbes, entirely disbe- 

 lieve in the change of species. When we see the forma- 

 tions tabulated in written works, or when we follow them 

 in nature, it is difficult to avoid believing that they are 

 closely consecutive. But we know, for instance, from Sir 

 R. Murchison's great work on Kussia, what wide gaps there 

 are in that country between the superimposed formations ; 

 so it is in North America, and in many other parts of the 

 world. The most skilful geologist, if his attention had 

 been confined exclusively to these large territories, would 

 never have suspected that during the periods which were 

 blank and barren in his own country, great piles of sedi- 

 ment, charged with new and peculiar forms of life, had 

 elsewhere been accumulated. And if, in every separate 

 territory, hardly any idea can be formed of the length of 

 time which has elaosed between the consecutive formations, 

 we may infer that this could nowhere be ascertained. The 

 frequent and great changes in the mineralogical composition 

 of consecutive formations, generally implying great changes 

 in the geography of the surrounding lands, whence the sedi- 

 ment was derived, accord with the belief of vast intervals 

 of time having elapsed between each formation. 



We can, I think, see why the geological formations of 

 each region are almost invariablv intermittent; that is, have 



