GEOGRAPHICAL DISTRIBUTION. ooo 
CHAPTER IV. 
GEOGRAPHICAL DISTRIBUTION OF CORAL REEFS AND 
ISLANDS. 
THE distribution of coral reefs over the globe depends on 
the following circumstances, arising from the habitudes of 
polyps already explained. 
1. The temperature of the ocean. 
2. The character of coasts as regards (a) the depth of 
water; (b) the nature of the shores; (c) the presence of 
fresh-water streams ; (d) the direction and force of currents. 
3. Liability to exposure to destructive agents, such as 
volcanic heat. 
It has been stated (p. 108) that reef-growing corals will 
flourish in the hottest seas of the equator, and wherever the 
average temperature of the waters of the ocean for the winter 
is not below 68° F. The isothermal line of this temperature 
(or isocryme) forms, therefore, the boundary line of the coral- 
reef seas. Other corals not forming reefs grow in colder seas 
(p. 109) and at great depths, but to those we do not now refer. 
This line traverses the oceans between the parallels 26° 
and 30°, or in general near 28°. But, as has been stated, 
it undergoes in the vicinity of the continents remarkable 
flexures from the influence of oceanic currents, the polar cur- 
rents bending it toward the equator, while the tropical cause 
a divergence. From a comparison of the thermometrical 
