32 
shells of the adults by hundreds, the number of half-grown- 
oysters is always less than one-half the number of adults. 
The great mortality of the young, after they have fastened 
themselves to the shells of the adults, is due in part to vrant 
of room, in part to the attacks of enemies, in part to acci- 
dents, such as the shifting of the bottom, and in part, no 
doubt, to lack of food. While the supply of organic matter 
which is carried to them by the water is very great, it is not 
unlimited, and the amount which each oyster can obtain at 
any one time is quite small, and if the oysters covered the 
bottom in sufficient abundance, some of them might fail to 
obtain a sufficient supply. I do not believe, however, that 
this ever occurs, for long before the oysters are sufficiently 
abundant to exhaust the supply of organic matter, their num- 
bers are limited by other conditions. The growth of an ani- 
mal does not depend upon the supply of food in general, 
but upon the supply of the least abundant of the ne- 
cessary ingredients of the food. It is well known that 
a field that is very fertile will fail to produce a satisfactory 
crop of a plant which needs some particular food-ingredient 
which the soil contains in too small quantity. Although food 
in general is very abundant, the growth of this particular 
crop depends upon the amount of this ingredient, and while 
the seed which has been planted yields an abundant crop of 
young plants, only a few are able to grow up, and these can 
grow no faster than they can extract this particular ingredi- 
ent from the soil. 
In addition to organic food, the oyster needs a supply of 
carbonate of lime to make its shell, and this is supplied to it, 
in solution, in sea-w T ater. If the shell is thin, or if it is formed 
very slowly, the danger from enemies and accidents is greatly 
increased ; and those oysters which are able to construct their 
shells with the greatest rapidity are the ones which survive 
and grow up. The amount of dissolved carbonate of lime 
which the ocean contains is unlimited, but the amount which 
can reach each oyster is not very great ; and if all the oysters 
which attach themselves were to survive there can be no doubt 
that they would exhaust the available supply of lime before 
they failed to obtain enough organic food. 
