It is highly probable that the ability to distinguish between living and nonliving objects was already well developed in early prehuman animals. Cognizance of the difference between these two classes of objects, long a part of human knowledge, led naturally to the division of science into two categories: physics and chemistry on the one hand and biology on the other. So deep was this belief in the separateness of physics and biology that, as late as the early nineteenth century, many biologists still believed in vitalism, according to which living phenomena fall outside the confines of the...