204 GEOLOGICAL DISTRIBUTION". 



Because they became extinct at a given period of time in one region 

 is no reason why they should have faded out during the same period 

 in every other; nothing could be more illogical than such an as- 

 sumption, and the facts, in numerous parallel cases, clearly demon- 

 strate its utter fallaciousness. Then why lay such stress upon the 

 occurrence of these animals as an indication absolute of geological 

 time ? Every day's lesson teaches the geologist how unstable are the 

 limits that have been assigned by him to the duration of life in 

 species, or groups of species ; it ought to be, therefore, a matter of 

 no surprise, but the reverse, to find certain so-called " distinctive " 

 or " characteristic " forms becoming less and less distinctive with 

 the progress of investigation. The discovery of dinosaurian re- 

 mains in Tertiary deposits, or of trilobites in the Permian, should 

 give far less cause for surprise than a positive announcement that 

 they did nowhere so occur. 



What the proximate cause of the extinction of species or groups 

 of species may have been, it is in most cases impossible to de- 

 termine. The process is such a gradual one, and its manifesta- 

 tion so casual, that we fail to see what it is just exactly that acts. 

 If, as Darwin puts it, "we ask ourselves why this or that spe- 

 cies is rare, we answer that something is unfavourable to its con- 

 ditions of life; but what that something is, we can hardly ever 

 tell. On the supposition of the fossil horse still existing as a rare 

 species, we might have felt certain, from the analogy of all other 

 mammals, even of the slow-oreeding elephant, and from the his- 

 tory of the naturalisation of the domestic horse in South America, 

 that under more favourable conditions it would in a very few years 

 have stocked the whole continent. But we could not have told 

 what the unfavourable conditions were which checked its increase, 

 whether some one or several contingencies, and at what period of 

 the horse's life, and in what degree, they severally acted. If the 



necessary correlation of strata impossible. At Laredo, on the Eio Grande, 

 Texas, the Claiborne beds (Parisian) are stated by Professor Cope to rest " im- 

 mediately on tbe Laramie" (" Proc. Am. Pliil. Soc," 1884, p. 615). If tbis 

 is really the ease, then it is more tlian likely that the latter is at least in part 

 the equivalent of the basal Tertiary, otherwise it would he difficult to account 

 for the sudden disappearance here of the vast thickness of sub-Claibomian 

 deposits (Buhrstone ; Eo-Lignitic), measurinar hundreds of feet, which else- 

 where along the Atlantic and Gulf borders forms the base of tlie Tertiary 

 series. 



