MODE OF FORMATION. 103 
made by those residing in coral seas, either in the manner adopted by 
Mr. Allan, or more definitely by placing marks upon particular species. 
By inserting slender glass pins a certain distance from the summit of 
a Madrepore, its growth might be accurately measured from month 
to month. ‘T'wo such pins in the surface of an Astrea, would in the 
same manner, by the enlarging distance between, show the rate of 
increase in the circumference of the hemisphere; or if four were 
placed so as to enclose an area, and the number of polyps counted, the 
numerical increase of polyps resulting from budding, might be ascer- 
tained. Itis to be hoped that some of the foreign residents at the 
Sandwich, Society, Samoan or Feejee Islands will take this subject 
in hand. ‘There are also many parts of the West Indies, where these 
investigations might be conveniently made. 
IIL FORMATION OF REEFS, AND CAUSES OF THEIR FEA- 
TURES AND GEOGRAPHICAL DISTRIBUTION. 
An inquiry into the causes and origin of the features presented by 
coral reefs and islands, has led us to glance at the nature of coral- 
zoophytes, and at the effects of various agents upon their development. 
The way has thus been prepared for considering the bearing of these 
facts, and of other influencing causes, on the growth of the coral plan- 
tation as a whole. While, therefore, the preceding pages treat of 
zoophytes as individual species, the following will relate to those 
results which proceed from their accumulation, and the causes 
which have determined the features and geographical distribution of 
reefs and islands. 
1. FORMATION OF REEFS. 
Very erroneous ideas prevail, respecting the appearance of a bed or 
area of growing corals. The submerged reef is often thought of as 
an extended mass of coral, alive uniformly over its upper surface, and, 
by this living growth, gradually enlarging upward: and such precon- 
ceived views, when ascertained to be erroneous by observation, have 
sometimes led to scepticism with regard to the zoophyte origin of the 
reef-rock. Nothing is wider from the truth: and this must have 
been inferred from the descriptions already given. Another glance 
