naturally supposed to be distinct kinds of Batrachians; but the experiments of Prof. Marsh and Prof. Dumeril have demonstrated that changing the conditions of existence has the effect of metamorphosing the Siredon into an Amblystoma. The importance of these experiments may be appreciated by supposing that Tadpoles reproduced Tadpoles in Nebraska or Mexico, and Frogs reproduced Frogs in New Haven or Paris, and that anatomists regarded the Tadpole as an entirely distinct Batrachian from the Frog; but removing the Tadpoles to New Haven or Paris, and changing the conditions of existence, the Tadpole turned into a Frog: our hypothetical case is exactly that of the Siredon and Amblystoma. As Professors Marsh and Dumeril developed a lung-breather, the Amblystoma, from a gill-breather, the Siredon, so we believe nature to have developed the lung-breathing Frogs from gill-breathing tadpole-like animals. Thus, in remote time, the conditions of existence changing, some of these tadpole-like animals changed into Frogs, while others remained unmodified and reproduced tadpole-like animals. But as the development of the individual Frog is the epitomized history of the race to which it belongs, the developing Tadpole, or the transitional stages of the Frog, are permanently represented by animals like the Salamander and Proteus.
An important consequence of the Struggle for Existence is the Division of Labor so characteristic of man, the higher animals, and plants. Savages supporting themselves by hunting and fishing, while often acting in concert, are, however, not dependent upon one another, so that the sudden death of even many individuals does not cause any inconvenience to the rest of the tribe. But in the civilized state, where the crowding together of people diminishes the means of subsistence and increases the number of rivals, the Struggle for Existence soon differentiates the population into growers of corn, hewers of wood, and carriers of water,
