166 



CHARLES B. WARRING, M.A., PH.D., ON 



species of which we have any record."* I might multiply such 

 quotations, but there is no need. 



A very important fact in this connection is that after these 

 exterminations the old species do not reappear, but new ones, 

 more like those of to-day , take their places. The biological 

 conditions, therefore, must have changed in the interval, 

 probably a long one, between the birth and death of those 

 species, and always in the direction of those that now prevail ; 

 and as this change of species took place all over the world, an 

 explanation, must be sought in a cause, or causes, possessing the 

 same characteristics of permanency and universality. 



It is usual to attribute these exterminations to the occurrence 

 of continental elevations producing colder climate, or to high 

 latitude depression of land letting cold waters from circumpolar 

 regions flow towards the equator. It is doubtful whether these 

 were sufficiently extensive to be world-wide, and, however that 

 maybe, they lack permanency ; for in a short time, geologically 

 speaking, the elevations were followed by depressions, and the 

 parts that had been sunk below the normal level came up 

 again ; and as for cold water currents, if we may judge from 

 the flora and fauna which have been preserved, circumpolar 

 waters during by far the greatest part of the time when the 

 exterminations occurred differed in temperature but little, if 

 at all, from those within (or at least near) the tropics. It is 

 found that from the Eozoic to the close of the Mesozoic, and in 

 a less degree through the Miocene, one of the most striking 

 characteristics of each horizon was the world-wide prevalence 

 of very similar, and often identical species, with little or no 

 regard to differences of latitude. 



It was during that time of mild and uniform climate that 

 the most numerous and most remarkable exterminations took 

 place. There may have been local elevations sufficient to 

 produce even large glaciers — mountains in tropical regions 

 have such. now — but their influence in the early days was too 

 limited to need to be considered in this connection. We must, 

 therefore, look elsewhere for causes which were both world- 

 wide and permanent, and which rendered a return to former 

 biological conditions impossible. These we shall find, if I 

 mistake not, in the continuous improvement from the dawn of 

 life in the character of the atmosphere, the waters, and the 

 soil. Its influence was world-wide, never going back, perma- 



* Man. of Geol., page 487, near bottom. 



