In like manner, the ascidians, or sea-squirts--mollusks of low grade, or, as Haeckel prefers to regard them, allied to worms--are most remote in almost every respect from the vertebrates. But in the young state of some of these creatures, and in the adult condition of one animal referred to this group (_Appendicularia_), they have a sort of swimming tail, which is stiffened by a rod of cartilage to enable it to perform its function, and which for a time gives them a certain resemblance to the lancelet or to embryo fishes; and this usually temporary contrivance--curious as an imitative adaptation, but of no other significance--becomes, by the art of "appearing and disappearing," a rudimentary backbone, and enables us at once to recognize in the young ascidian an embryo man. A second method characteristic of the book, and furnishing, indeed, the main basis of its argument, is that of considering analogous processes as identical, without regard to the difference of the conditions under which they may be carried on. The great leading use of this argument is in inducing us to regard the development of the individual animal as the precise equivalent of the series of changes by which the species was developed in the course of geological time. These two kinds of development are distinguished by appropriate names. _Ontogenesis_ is the embryonic development of the individual animal, and is, of course, a short process, depending on the production of a germ by a parent animal or parent pair, and the further growth of this germ in connection more or less with the parent or with provision made by it. This is, of course, a fact open to observation and study, though some of its processes are mysterious and yet involved in doubt and uncertainty. _Phylogenesis_ is the supposed development of a species in the course of geological time and by the intervention of long series of species, each in its time distinct and composed of individuals each going regularly through a genetic circle of its own.