There is an amphibious animal, called the triton or water-salamander, akin to the frog, whose normal course is to begin life living in the water and breathing by gills, and end it on land with gills metamorphosed into lungs. If they are shut up in water and kept in a tank they never lose their gills, but continue through life in the lower stage of development, and reproduce themselves in other tritons with gills. Conversely the axolotl, a peculiar gilled salamander from the Lake of Mexico, has its normal course to live, die, and propagate its species in water, breathing by gills; but if an axolotl happens to stray from the water and take to living on dry land, the gills are modified into lungs and the animal gains a place in the class in the school of development. This fits in remarkably with the fact that the embryo of all vertebrate mammals, including man, passes through the gilled stage before arriving at the development of lungs, which assists us in understanding two facts of primary importance in the history of evolution. First, how terrestrial life may have arisen from aquatic life by adaptation to altered conditions. Secondly, how the evolution of the embryo sums up in the individual, in the period of a few days or months, the various stages of evolutions which it has taken millions of years to accomplish in the species. As a parallel to the transformation of gills into lungs, and of an aquatic into a land animal, if we turn to the geological records of the Secondary period we may trace the transformation of a water into an air population, of sea-lizards into flying-lizards, and of flying-lizards into birds. The ‘Hesperornis’ is an actual specimen of the transition, being a feathered lizard, or rather winged and feathered creature which is half lizard and half bird. A remarkable instance of the great change of functions which may be produced by a change of outward conditions is afforded by the common ringed snake, which in its natural state lays eggs which take three weeks to hatch; but if confined in a cage in which no sand is strewed it hatches the eggs within its own body, and from oviparous becomes viviparous. This may help us to understand how the lowest order of mammals, which, like the Australian echidna or duck-billed mole, lay eggs, may have developed, first into marsupial, and finally into placental mammals. These examples may assist the reader in understanding how the infinite diversities of living species may have been developed in the course of evolution from simple origins, just as the inorganic world was from atoms, by the action and reaction of primitive polar forces between the organism and its environment, and between heredity and variation. CHAPTER X. THE KNOWABLE AND UNKNOWABLE—BRAIN AND THOUGHT.