96 CORALS AND CORAL ISLANDS. 
alive over the whole surface, owing to a symmetrical and un- 
limited mode of budding, are nothing but lifeless coral 
throughout the interior. Could the living portion be sepa- 
rated, it would form a hemispherical shell of polyps, in most 
species about half an inch thick. In some Porites of the same 
size, the whole mass is lifeless, excepting the exterior for a 
sixth of an inch in depth. 
With such a mode of increase, there is no necessary limit 
to the growth of zodphytes. The rising column may increase 
upward indefinitely, until it reaches the surface of the sea, and 
then death will ensue simply from exposure, and not from any 
failure in its powers of life. The huge domes may enlarge till 
the exposure just mentioned causes the death of the summit, 
and leaves only the sides to grow, and these may still widen, 
it may be indefinitely. Moreover, it is evident that if the 
land supporting the coral domes and trees were gradually 
sinking, the upward increase might go on without limit. 
In the following of death after life ‘“ aquo pede,” there is 
obedience to the universal law. And yet the polyps, through 
this ever yielding a little by piecemeal, seem to get the better 
of the law, and in some instances secure for themselves almost 
perpetual youth, or at least a very great age. Of the polyps 
over an Astraea hemisphere, none ever die as long as the dome 
is in a condition of growth; and the first budding individual, 
or at least its mouth and stomach, is among the tens of 
thousands that constitute the living exterior of the dome of 
fifteen feet diameter. In the Madrepore, the terminal parent- 
polyp of a branch grows on without being reached by the 
death-warrant that takes off at last the commoners about the 
base of the tree; it keeps growing and budding, and the tree 
thus continues its increase. 
The death of the polyps about the base of a coral tree 
