The animal kingdom, no less than the vegetable, gives these results. Amphioxus, the little Mediterranean fish, links the Vertebrata, or back-boned animals, for ever on to the Invertebrata. The classes of the Vertebrate sub-kingdom have their connecting links or intermediate forms. These classes, adopting for popular exposition the old classification, are the Pisces, Amphibia, Eeptilia, Aves, Mammalia. Whilst Amphioxus at the lower end of the class of fishes connects these with the soft-bodied animals, or Mollusca, at the upper end of the Pisces, we have the Lepidosiren, or mud-fish. It is impossible to say whether this animal is more of a fish or a reptile. With limbs rather than fins, with three cavities to its heart, and a swim-bladder that acts as a lung, it has yet so many parts of its anatomy that are piscine as to lead Professor Huxley still to place it as a solitary representative of the highest order of Pisces. The class Amphibia is itself a confirmation of the general truth, for its members, such as the frogs, are in their early condition fish, and in their adult state reptiles. Pterodactyl of the Jurassic strata is the winged lizard. Its name tells us that we have a form intermediate between the classes Reptilia and Aves. The duck-billed Platypus, or Ornithorhyncus, of Australia, is a furred mammal that suckles its young, and yet has a bird's bill, a bird's feet, a bird's wishing-bone, a bird's heart, a bird's alimentary canal. If we turn to the individual classes, the same thing obtains. To take but the the highest class, the Prosimiae, or half-apes, among the Mammalia are an order, that stands centrally to the Insectivora, Eodentia, Cheiroptera, and Primates. There is no gap between man and the rest of the Primates. Not a single mark of anatomy, of physiology, or of psychology, clearly distinguishes man from the highest apes. If we study the individual animal, the same fact of the unity of phenomena is again borne in upon us. The bodily functions are by no means so distinct in their nature as we were wont to think. To take but an illustration. The sense-organs of man are all found to be only so many modifications of the integument. The skin or tactile organ is the integument. The tongue or taste organ is but the integument folded inwards and a little modified. The nasal cavities are also lined with a modification of the same tissue, and even the most complex sense organs that are at the same time the most important--that is the eye and the ear--are, as the study of development or embryology shows us, only the result of a series of remarkable changes affecting certain parts of the epidermis of the animal.