124 DYNAMICAL GEOLOGY. 



vented from spreading and intermingling by the impassa- 

 ble barrier of the ocean. Even apparent exceptions, 

 when examined, confirm the law, as we now proceed to 

 show. 



Fig. 68 is a north-polar view of the earth, and 1, 2, 3, 

 4, 5, are the temperature zones so often referred to. Now, 



FIG. 68 



commencing with Nos. 4 and 5, the species in the Eastern 

 and Western Continents are substantially the same, for 

 the lands of the two continents approach each other in 

 these zones so nearly that they may be considered as one. 

 There is no barrier to the spread of species all around the 

 pole. But when we come to No. 3, and still more to 

 No. 2, the difference of species is almost complete, and 

 many genera are also different and that, not because the 

 physical conditions are unsuitable ; for European species 

 introduced in our country do so well that they often kill 

 out our own native species. Nearly all useful and nox- 

 ious species have been thus introduced. They were not 

 here when America was discovered only because they 

 could not get here. 



