LIFE AND DEATH IN CONCURRENT PROGRESS. 95 
yps gradually disappear, and even their cells become superfi- 
cial and fade out. Trees of Madrepores may also have their 
limits—all below a certain distance from the summit being 
dead; and this distance will differ for different species. Bur 
this is not a limit to the existence of the zodthome, even 

CAULASTR#A FURCATA, D. 
though a slender tree or shrub, or of its flourishing state; for the 
dead coral below is firm rock itself, often stronger than ordinary 
limestone or marble, and serves as an ever-rising basement 
for the still expanding and rising zodphyte. 
But this death is not in progress alone at the base of the 
column orbranch. Generally the whole interior of a corallum 
is dead, a result of the same process with that just explained. 
Thus, a Madrepora, although the branch may be an inch in 
diameter, is alive only to the depth of a line or two, the grow- 
ing polyps of the surface having progressively died at the low- 
er or inner extremity as they increased outward. 
The large domes of Astrzeas, which have been stated to 
attain sometimes a diameter of ten or fifteen feet, and are 
