Though without nerve and special sense-organs, yet these little hollow Gastrœada, and, in fact, their ancestors, the Amœbæ, which consisted of simple protoplasmic cells, each enclosing a nucleus, were possessed with sensory perception, being influenced by light, and by variations of pressure and temperature. As the evolutionary process continued, and the animal kingdom assumed higher forms, the original epidermal general sense-organ became converted into several special organs of sense, each specialisation commencing with a simple depression upon the integument of the organism; numerous little epidermal nerves of perception were formed, which could perceive changes of pressure and of temperature, and some of which gradually became enabled to understand particular influences affecting them, such as those produced by a strong odour, light-waves, and sound-waves. By adaptation, the extremities of these sense-nerves became expanded and enlarged, so as to enable them the better to understand the particular influences; and this expansion was accompanied by a corresponding depression on the integument, which cup-like formation afterwards became converted into an eye, or other organ of special sense, very imperfect in the invertebrate forms of life, imperfect in the fish, more perfect in the amphibian, and still more perfect in the mammal forms, such as apes and men. In short, the life-history of the individual is an exact counterpart in miniature of the life-history of the species up to the particular point reached by the particular individual.