n] FEATURES OF ENVIRONMENT 53 



2. Temperate species are more powerfully in- 

 fluenced by ascent into the cold zone, than by descent 

 into the hot lands. 



3. The change into a colder environment is a 

 more powerful factor than change into a warmer 

 climate. 



The Mexican Biota is composed of (1) endemics, 

 (2) nearctics, immigrants from the north, (3) neo- 

 tropical immigrants from the south. Of these the 

 nearctics are more long-ranged than the tropicals, 

 i.e. they are more hardy, in the sense of being re- 

 sistant, dominant, able to accommodate themselves 

 to new environment without becoming changed. The 

 short-ranged among the nearctics are mostly those 

 which have become modified for cold life. 



Since the majority of the genera and species of 

 tropical or southern origin are short-ranged, this 

 means either that they are softer, more pliable, re- 

 cessive ; or that they are more influenced by ascent. 

 Species B at the bottom has changed into B 1 in the 

 middle region, and into B 2 towards the top ; hence 

 three short-ranged species. A hot land or bottom 

 species can obviously change only into cooler climes, 

 but an originally temperate species may ascend or 

 descend. 



If the three rules enumerated above hold good, 

 it seems paradoxical that denizens of the hot lands 

 transported into a cold place stand this change much 



