I7& THE SUCCESSION OF ANIMAL FORMS 



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 expansion were 

 eminently those of introduction of new specific types, and 

 periods of contraction those of extinction, and also of continu- 

 ance of old types under new varietal forms. 



It must also be noticed that all the leading types of in- 

 vertebrate life were early introduced, that change within these 

 was necessarily limited, and that elevation could take place 

 mainly by the introduction of the vertebrate orders. So in 

 plants, Cryptogams early attained their maximum as well as 

 Gymnosperms, and elevation occurred in the introduction of 

 Phsenogams, and this not piecemeal, but as we shall see in 

 a succeeding chapter, in great force at once. 



We may further remark the simultaneous appearance of like 

 types of life in one and the same geological period, over widely 

 separated regions of the earth's surface. This strikes us es- 

 pecially in the comparatively simple and homogeneous life- 

 dynasties of the Palaeozoic, when, for example, we find the same 

 types of Silurian Graptolites, Trilobites and Brachiopods ap- 

 pearing simultaneously in Australia, America and Europe. 

 Perhaps in no department is it more impressive than in the 

 introduction of the Devonian and Carboniferous Ages of that 

 grand cryptogamous and gymnospermous flora which ranges 

 from Brazil to Spitzbergen, and from Australia to Scotland, 

 accompanied in all by the same groups of marine invertebrates. 

 Such facts may depend either on that long life of specific 

 types which gives them ample time to spread to all possible 

 habitats, before their extinction, or on some general law where- 

 by the conditions suitable to similar types of life emerge at one 

 time in all parts of the world. Both causes may be influential, 

 as the one does not exclude the other, and there is reason to 

 believe that both are natural facts. Should it be ultimately 

 proved that species allied and representative, but distinct in 



