Well, we might ask the same question of men, or horses, or birds, or any other living creature, as of trees. The reason is that the growth of living things is not like the growth of a snowball or a crystal. Such things simply go on growing as the more “food” they get. There is nothing inside them limiting their growth. But every living creature has the marks within it of something that has a purpose – which snowballs and crystals have not – and that purpose is simply to live. In general we may say that living things grow to the size which suits them best, and so favors their life best. When that is done, they stop growing, and merely maintain themselves as long as they can.
We have to think of every living creature as if it had a plan, a definite course of action which it had resolved upon, and which it will pursue if the conditions outside allow it to do so. It grows to the shape and size which it is meant to grow to, and then no powers on Earth, no amount of food and air and light, will make it grow even the least little bit more.
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