476 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



northern limit still unmistakably the same. They do 

 not, and apparently can not, change their specific char- 

 acters. They die first. (2) Take another example, the 

 sweet gum (liquidambar) of the eastern coast. This 

 remarkably distinct form ranges from Florida to the 

 borders of the Great Lakes. Throughout all this wide 

 range it is precisely the same unmistakable species, 

 characterized by its peculiar starred leaf, winged twigs, 

 spinous burr, and fragrant gum. On the limits of its 

 range it does not change into any other species, but sim- 

 ply dies out and is replaced by others. 



These are illustrations of what has been called "per- 

 manence of specific form." It is as if species originated, no 

 matter how — say, at once by creation — in their present 

 form, somewhere in the region where we find them, and 

 thence spread in all directions as far as physical condi- 

 tions and struggle for life with other species would allow, 

 interlocking there with other contesting species and co- 

 existing with them on the common border. 



4. Barriers. — Faunas and floras shade gradually 

 into one another when limited only by temperature con- 

 ditions; but if there should be an impassable physical 

 barrier, like an east and west mountain range, or a des- 

 ert or sea, then the fauna and flora on each side of such 

 barrier will differ greatly and without any shading. Thus 

 the species north and south of the Himalaya, or north 

 and south of the Sahara, differ conspicuously and tren- 

 chantly. It is again as if each group of organisms had 

 originated or been created at once out of hand just as 

 they are and where we find them, and spread as far as 

 they could, but could not mingle with the next contigu- 

 ous group on account of the intervening barrier. 



5. North and South of the Equator. — Again, 

 there are temperature zones south of the equator as well 

 as north ; but none of the species found in temperate or 



