Aristotle's Psychology: On the Soul and Parva Naturalia

continuity in terrestrial life. The initial form of this is found in plant-life. The plant-organism is simpler than any other, its functions are confined to nutrition and reproduction. The function of growth or vegetation in plants is analogous to the nutritive functions in higher organisms. A process of conversion and assimilation is carried on in both cases and by analogous organs. Roots are analogous to the mouths of animals,¹ or, as Aristotle elsewhere employs another analogy, they are like umbilical veins that take in nourishment from the earth as the embryo is maintained by its attachment to the uterus.² Plants, furthermore, as Aristotle observed, exhibit the morphological tendency to develop their organs at the extremities, while animals tend to develop theirs at the centre.³

The transitional form of life in proceeding from plants to animals, or from phenomena of growth to phenomena of sensation, is found in the Zoophytes. There are some marine animals, Aristotle says,⁴ concerning which it is difficult to say whether they are plants or animals, for many of them grow on rocks and die if detached. To these transitional forms belong the sponges, holothurians, star-fishes, acalephae sea-anemones, and sea-lungs.⁵ All of these possess a low degree of sensation, and some of them are incapable of movement. Aristotle's reason for classifying sponges amongst animals seems to have been