But if, of a group of weaker forms, those alone survive which adopt a parasitic life, those which in adult life move the least will survive and reproduce; there will result the survival of the least muscular and nervous. This degeneration will continue until the species has sunken into equilibrium, so to speak, with its surroundings. Here natural selection works for degeneration. Sessile animals have had a similar history. But these parasitic and sessile forms had already been hopelessly distanced in the race for life. Their presence cannot impede the leaders; indeed their survival is necessary to directly or indirectly furnish food for the better conformed. In the animal and plant world there is abundant room and advantage at the top. Once more, natural selection works as a rule for the survival of individuals, only indirectly for that of organs composing, or of species including, these individuals. It may work for the development of a trait or structure which, while of no immediate advantage to the individual, increases the probability of its rearing a larger number of fitter offspring. Thus defence of the young by birds may be a disadvantage to the parent, but this is more than counterbalanced in the life of the species by the number of young coming to maturity and inheriting the trait. Even here natural selection favors the survival of the trait indirectly by sparing the descendants of the individual possessing it. Natural selection may always work on and through individuals without always working for their sole and selfish advantage. In human society we find the selection of families, societies, nations, and civilizations going on, but mainly as the result of the survival of the fittest individuals. There may very probably be a struggle for existence between organs or cells in the body of each individual. The amount of nutriment in the body is a more or less fixed quantity; and if one organ seizes more than its fair share, others may or must diminish for lack. But the limit to this usurpation must apparently be set by the crowding out of those individuals in which it is carried too far. Natural selection, so to speak, leaves the individual responsible for the distribution of the nutriment among the organs, and spares or destroys the individual as this usurpation proves for its advantage or disadvantage. It makes its verdict much as the judges at a great poultry or dog show count the series of points, giving each one of them a certain value on a certain scale, and then award the prize to the individual having the highest aggregate on the whole series. Any such illustration is very liable to mislead; I wish to emphasize that fitness to survive is determined by the aggregate of the qualities of an individual.