From the book:
Childhood is a stage in the process of that continual remanufacture of the Life Stuff by which the human race is perpetuated. The Life Force either will not or cannot achieve immortality except in very low organisms: indeed it is by no means ascertained that even the amoeba is immortal. Human beings visibly wear out, though they last longer than their friends the dogs. Turtles, parrots, and elephants are believed to be capable of outliving the memory of the oldest human inhabitant. But the fact that new ones are born conclusively proves that they are not immortal. Do away with death and you do away with the need for birth: in fact if you went on breeding, you would finally have to kill old people to make room for young ones