122 DYNAMICAL GEOLOGY. 



vigorous in the middle region it may decrease in vigor 

 and number on its borders ; but in all specific characters, 

 wood, bark, leaf, and burr, it is the same throughout. 

 The study of species, as they now are, would probably not 

 suggest, certainly could not prove, the theory of their 

 origin by derivation or transmutation. 



4. Temperature regions shade into each other. But 

 this is so only where no barriers exist. If there be barriers, 

 such as an east and west mountain-chain, or sea, or desert, 

 then on the two sides of the barrier the species will be 

 very distinct and without gradation by overlapping. 

 Thus, north and south of the Sahara, and north and 

 south of the Himalayas, there is a marked and, as it were, 

 a sudden change of species. It is, again, as if the species 

 originated each in its own area and spread, but were pre- 

 vented from mingling and overlapping on their borders 

 by the barrier. 



5. Again, although there are similar temperature re- 

 gions on tropical mountains and in high latitudes and 

 these latter are also repeated north and south of the 

 equator yet the species are always different in the three 

 cases. This is because the torrid zone is a barrier pre- 

 venting migration. It is, again, as if species originated 

 each in its own place, but were prevented from reaching 

 similar temperature regions elsewhere by the existence of 

 impassable barriers. 



Zoological Temperature Regions. Animal species 

 are limited by temperature, like plants, and therefore also 

 exist in temperature zones ; but they can not be arranged 

 in the same simple way, evident even to the popular eye 

 i. e., great classes corresponding to great zones. It is 

 true that, if we compare extremes, viz., polar with tropi- 

 cal regions, we find a conspicuous contrast determined by 

 temperature, certain great families being characteristic 

 of each as, for example, among mammals, the great 

 pachyderms, the elephant, rhinoceros, hippopotamus, and 



