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 Aretics. 
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 Lichens of 
