If, however, acquired characters are not hereditary the method of natural selection in racial progress is curiously indirect. Apart from the preservation of their fecundity, the cells on which the continuity of life, in all the higher animals, depends, have themselves taken little part in the struggle for existence. Just as in the forest tree, the firmly implanted roots, the sturdy stem, and the strong branches have to bear the stress of the winter storm, that the flowers of spring may ripen the seeds which contain the potentiality of all this strength; so do muscle, sinew, and brain secure the survival of the animal, that his descendants may carry on the struggle. One may liken the cellular constituents of the animal to a hive of bees with fertile drones and queen, and sterile workers. It is on the exertions of the latter that, in the struggle for existence, the continued existence of the swarm depends, while it is by the pairing of the fertile drone and queen that the continuity of the race is secured. No worker can transmit the qualities which are so essential to the well-being of the community. But in the eggs of their sister the queen-mother these qualities lie dormant. And since the race is one race, the workers by their exertions contribute indirectly to the maintenance of those hereditary aptitudes to which they are unable to contribute directly. For it is essential to bear in mind that they not only work for their own generation, but they determine the course of heredity. Picture two such communities set in an environment which intensifies the struggle for existence. The one is strong, healthy, and vigorous; the other in all respects the reverse. The incidence of the battle of life falls mainly on the workers. If they succumb in the one group their fertile queen either perishes, or gives rise to a poor stock, certain in the long run to be eliminated. But the vigorous workers in the other group survive and secure, too, the survival of their queen, who, since she is also their sister, bears, in her ovaries, the good seed from which a new generation of vigorous workers shall be developed. Thus though the sterile bees contribute nothing directly to the heredity of the race, they indirectly determine the direction which that heredity shall take. So, too, in the higher animals, the reproductive cells are the fertile sisters of a host of sterile body cells, on which the main incidence of the struggle for existence falls. Their sterility precludes their directly contributing to the success of future cell-generations; but in protecting their fertile sisters, the reproductive cells, they are really determining the lines along which the evolution of the race shall continue.