In order to understand something of the complexity of this process, let us select a specific example. Among marine animals, the oysters are remarkable for the immense numbers of eggs which they produce—the average for the American oyster is probably about 16,000,000. If all the progeny of a single oyster were to live and reproduce, and their progeny were to do likewise, and so on until there were great-great-grandchildren, the total number of oysters that were descendants of the original pair would be about 66,000,000,000,000,000,000,000,000,000,000,000 and their shells would make a mass eight times as great as the earth.
Now it is quite obvious that the earth cannot hold, and cover with water, a mass of oyster shells eight times as great as itself; the oceans, if they were spread evenly over the surface (which they never were, and never can be), would accommodate but a few of the great horde. Neither do those same oceans contain enough food to satisfy, or begin to satisfy, the needs of these theoretical descendants of a single oyster. Clearly, therefore, space and food alone are enough to prevent the undue multiplication of creatures upon the earth.
But there are factors other than space and food which aid in accomplishing the result. There are water conditions, animal enemies such as the starfish, and a host of other means by which the population of oysters is kept down. And even if it were to increase greatly, the numbers of starfish would at the same time increase, and simultaneously set about decreas-
