I hold in my hand here a key to one of the greatest mysteries of life: the perennial mystery of birth and reproduction. It is only a pea that I hold here before me, an ordinary small, round, yellow marrowfat, the seed of the commonest of garden annuals. Nevertheless, that familiar little object, which all of us have known all the days of our life, incloses in itself the entire solution of the riddle of birth. If we understand the pea clearly, we understand the whole science of biology. Let us ask ourselves first, exactly what it is, and then see how it helps us to comprehend the coming into existence of all the higher plants and animals. We have here the solution for one of the deepest and most fundamental problems of all life, animal or vegetable: the problem of reproduction, heredity, and individual variation.