structure among the members of a group consist in different modifications of a fundamental type. Among the vertebrates the fore-limb may be an arm, a leg, a wing, a shovel, a flipper, or a fin. But in all cases it is the same organ—that is, the same implement modified to serve different ends. Take the mouth-parts of insects. In the grasshopper and cricket these parts are fitted for grinding; in the moths and butterflies they are fashioned into long tubes for sucking the sweets of flowers; in the mosquito they form an elaborate apparatus for drilling and drinking; and in the mayfly the mouth-parts, though present, are not used at all. In all of these animals these parts are essentially the same, although differing so much in their forms and purposes that the unscientific can scarcely be made to believe they are fundamentally alike. There is no fact more familiar to the biologist or more frequently met with in the fields of animal morphology than the fact that the same general type may be hammered into dozens, or hundreds, or even thousands, of different patterns by the incessant industry of its surroundings, and that the same organic part may be moulded into various implements serving totally different ends by the environmental vicissitudes of time and space. On the hypothesis that the members of each group of animals possessing common characteristics, whether the group be large or small, have sprung from a common ancestry, and that the differences in structure have arisen as a result of differences in environment, the similarities and homologies of structure existing among animals are perfectly intelligible. But on any other supposition they are inexplicable. 3. Evolution is suggested by the remarkable series of phenomena presented by embryology. There are at least four facts in the developmental history of every creature which can hardly be accounted for on any other supposition than that of organic evolution. _First_, the fact that every animal, above the lowest, individually passes through an evolution between the beginning of its existence and its maturity. Terrestrial beings are not born, like Minerva, full-grown. They grow. They evolve. They commence close down to the very atoms. And from this lowly genesis they rise, through a series of marvellous changes, to that high state of perfection and greatness from which they descend to dissolution.