class). When it has evolved to this higher class, it then passes from the lower order (“siren” order) to a higher one (“triton” order), and then to the highest order (“frog” order). The bi-concave vertebræ of the fish-like tadpole have now developed into vertebræ with the cup-and-ball joints of the higher amphibian. It is the same with all the complex organs of the adult frog; they evolve from the much simpler structures of the tadpole. [Illustration: FIG. 17.—Tadpoles and Frog; a, tadpole with branching external gills; b, gills absorbed and hind legs have appeared; c, fore legs have appeared; d, tail shrunk and legs enlarged; e, perfect, young frog,—tail entirely disappeared. The figures represent some stages in the life history of the frog.] This study of the frog’s evolution from the fertilized egg is profoundly instructive. It reveals to us, through direct observation, that a creature varies in its form and structure at succeeding intervals of time. These variations diverge more and more, so that specific, generic, and even ordinal and class distinctions are revealed as the development proceeds. Owen, the distinguished comparative anatomist, in speaking of the transmutation of one species into another in the course of geologic history, says, though with a hostile purpose in view, that in the metamorphoses of the amphibians we seem to have such process carried on before our eyes to its extremest extent. Not merely is one specific form changed to another of the same genus; not merely is one generic modification of an order substituted for another, the transmutation is not even limited by passing from one order (Urodela) to another (Anura); it affects a transition from class to class. The fish becomes the frog (amphibian); the aquatic animal changes to the terrestrial one; the water-breather becomes the air-breather; an insect diet is substituted for a vegetable one. And these changes, moreover, proceed gradually, continuously, and without any interruption of active life. Such is the language of Owen in reference to these remarkable transmutations of the developing frog.