FORMATION OF CORAL REEFS. 155 



found in all parts of the globe, yet the Reef- 

 Building Polyps are limited to the Tropics. We 

 are too apt to forget that the homes of animals 

 are as definitely limited in the water as on the 

 land. Indeed, the subject of the geographical 

 distribution of animals according to laws regu- 

 lated by altitude, by latitude and longitude, by 

 pressure of atmosphere or pressure of water, by 

 temperature, light, &c., already alluded to in a 

 previous article, is exceedingly interesting, and 

 presents a most important field of investigation. 

 The climatic effect of different levels of al- 

 titude upon the growth of animals and plants 

 is the same as that of different degrees of lati- 

 tude ; and the slope of a high mountain in the 

 Tropics, from base to summit, presents in a 

 condensed form, an epitome, as it were, of the 

 same kind of gradation in vegetable growth that 

 may be observed from the Tropics to the Arctics. 

 At the base of such a mountain we have all the 

 luxuriance of growth characteristic of the tropi- 

 cal forest, the Palms, the Bananas, the Bread- 

 trees, the Mimosas ; higher up, these give way 

 to a different kind of growth, corresponding to 

 our Oaks, Chestnuts, Maples, etc. ; as these wane, 

 on the loftier slopes comes in the Pine forest, 

 fading gradually, as it ascends, into a dwarfish 

 growth of the same kind ; and this at last gives 

 way to the low creeping Mosses and Lichen? of 



