256 THE CHAIN OF LIFE. 



Pvvery great Palaeozoic limestone, for example, indicates a 

 depression with succeeding elevation. On each elevation 

 marine animals were driven back into the ocean, and on each 

 depression swarmed in over the land, reinforced by new 

 species, either then introduced or derived by migration from 

 other localities. In like manner on every depression, land 

 plants and animals were driven in upon insular areas, 

 and on re- elevation again spread themselves widely. Now I 

 think it will be found to be a law here that periods of expan- 

 sion were eminently those of introduction of new specific types, 

 and periods of contraction those of extinction, and also of 

 continuance of old types under new varietal forms. It must 

 also be borne in mind that all the leading tyj)es of invertebrate 

 life were early introduced, that change within these was neces- 

 sarily limited, and that elevation could take place mainly by 

 the introduction of the vertebrate orders. Sc i»^ plants, Cryp- 

 togams early attained their maximum as well as Gymnosperms, 

 and elevation occurred in the introduction of l^hiiinogams. 



Another allied fact is the simultaneous appearance of like 

 types of life in one and the same geological ])eriod, over 

 widely separated regions of the earth's surface. This strikes 

 us especially in the comparatively simple and homogeneous 

 life-dynasties of the Pakiiozoic, when for exami)le we find the 

 same ty])es of Silurian Graptolites, 'i'rilobites and Brachiopods 

 appearing simultaneously in Australia, America, and ICurope. 

 Perhaps in no department is it more impressive than in the 

 introduction in the Devonian and Carboniferous ages of that 

 grand cryjitogamous and gymnospermous flora which ranges 

 from Brazil to Spitzbergen, and from Australia to Scotland, 

 accompanied in all by the same groups of marine invertebrates ; 

 or in the like wholesale })roduction of modern types of trees in 

 the Cretaceous. Such facts may depend either on that long 

 life of sijecific ty\)iii which gives them am))le time to s])read 

 to all possiJ)le habitats, before their extinction ; or on some 

 general law whceby the conditions suitable to similar types of 



