100 GRADATION AMONG ANIMALS. 



animals are not scattered over the surface of our 

 globe at random, but that they are associated to- 

 gether in what are called fauna, and that these 

 faunae have their homes within certain districts 

 called by naturalists zoological provinces. The 

 limits of these provinces are absolutely fixed, in 

 the ocean as well as on the land, by certain phys- 

 ical conditions connected with climate, with alti- 

 tude, with the pressure of the atmosphere, the 

 weight of the water, etc. ; and this is true even 

 for animals of migratory habits, for all such mi- 

 grations are periodical, and have boundaries as 

 definite and impassable as those that limit the per- 

 manent homes of animals. There is a certain 

 series established by the relations between differ- 

 ent kinds of animals, as thus distributed over the 

 globe, agreeing with the gradation in their rank, 

 their growth, and. their succession in time ; the 

 law which distributes animals in adjoining faunae, 

 arid in accordance both with their relative superi- 

 ority or inferiority, and with the physical condi- 

 tions essential to their existence, being the same 

 as that which controls their structural relations, 

 their embryological development, and their suc- 

 cession in time. 



What, then, does this correspondence between 

 the Series of Rank, the Series of Growth, the 

 Series of Time, and the Series of Geographi- 

 cal Distribution in the life of animals teach us ? 



