up against insurmountable difficulties. This in- adequacy of Darwinism is the second point we brought out when we spoke of the vital impetus: to a theory we opposed a fact, we pointed out that the evolution of life occurred in certain definite directions. Now, are these directions imposed on life by the conditions in which it evolves? This would ii THE VITAL IMPETUS 93 amount to admitting that the modifications undergone by the, individual are handed down to his descendants, at least regularly enough to ensure, for instance, the gradual com- plication of an organ accomplishing the same function with ever greater precision. But the heredity of acquired charac- teristics is debatable, and, even supposing that it is observed, exceptional; once again it is a priori, and in order to meet the needs of the argument, that it is taken to be operating regularly. Let us attribute this regular transmissibility to the innate: we shall conform to experience and we shall say that it is not the mechanical action of external causes, but an inward impulse that passes from germ to germ through individuals, that carries life in a given direction, towards an ever higher complexity. Such is the third idea to be evoked by the image of the vital impetus. Let us go further. When one speaks of the progress of an organism or an organ adapting itself to more complex conditions, one means, more often than not, that the complexity of conditions imposes its form on life, as