mentation, reproduction, etc. The reasoning already employed leads us to conclude that these lower animals were the ancestors of the higher, and transmitted to them-the qualities which the two classes possess in common. For example, both the higher and lower animals possess an alimentary canal—a tube running through the body for the reception and digestion of food. We conclude that this alimentary canal was not obtained by the higher animals through external causes, but by inheritance from the lower animals.
We have, finally, to consider those qualities which man, and both the higher and lower grades of animals, possess in common with the very lowest animals. These lowest animals, consisting in respect to their physical characters simply of minute jelly particles, destitute of organization, agree with the higher animals only in their physiological properties. These are essentially only two, nutrition and reproduction. These, indeed, are the two absolutely fundamental and essential properties of any living organism. Without the one, the life of the individual ceases; without the other, the life of the species. From the biological point of view, the carrying out of these two functions of nutrition and reproduction is the sole end of existence of any living being. Animals differ from one another—they occupy a lower or higher place in the scale of life—according to the advantages of organization enabling them to carry out these functions. The special powers possessed by animals which at first sight seemed to be ends in themselves, are seen by a moment's reflection to be only subservient to these two great ends. In birds, for example, both those powers depending upon structural perfection, as flight, vision, song, and plumage, and those depending upon a highly developed nervous system, as the instincts of migration and nest-building, serve, in the end, wholly to better enable the animals to maintain their own life and that of their species—to carry out the functions of nutrition and reproduction. Thus, rapid flight and keen vision enable them to procure food; melodious notes and brilliant plumage are sexual attractions; while migration and nest-building are directly connected with nutrition and reproduction respectively. From these considerations it is seen that, to the biologist, the simplest animals—the animate jelly-particles—are beings of far higher rank than their physical simplicity would indicate, since they carry on the same life-processes that other animals do, only lacking parts and organs subserving the operation of these processes. It is, therefore, only to assume that like proceeds from like to suppose that from these simplest animals the higher forms received by the law of heredity the two powers of their being which all possess in common—nutrition and reproduction. The differences subsisting between these lowest animals and the higher
