food and starve. During migration the birds that are strongest on the wing reach the land whither they are flying, while the weaker perish on their course. Thus Natural Selection acts in two ways, _eliminating the unfit_ and _selecting the fit_, and there are, besides, two special modifications of Natural Selection which are called =Sexual Selection= and =Insect Selection=. The following analysis of Natural Selection may be useful to the reader, viz.: { Physical and Climatic Causes. { Elimination { Enemies. { { Competition. { Natural Selection { { Natural Selection Proper. { { { { { By Preferential { { { Mating. { Selection { Sexual Selection. { { { By Battle. { Insect Selection. =Elimination of the Unfit.= The elimination of the unfit takes place through the agency of physical and climatic causes and also through enemies, and competition of members of the same species. Elimination through the action of surrounding physical and climatic conditions is shown by the following facts: if certain tropical animals be transferred to sub-Arctic or even temperate regions they are unable to adapt themselves to the requirements of the new climatic conditions, and die sooner or later; many animals are killed if the fresh-water lake in which they live be invaded by the waters of the ocean; fishes which live at great depths in the sea and are, therefore, subject to great pressure, are killed when they are brought to the surface, on account of the expansion of the gases in their tissues; if the water where corals are living becomes too fresh, too muddy, or too cold, they will die. The change of climate to a much colder temperature at the close of the Jurassic Ages was probably the cause of the extinction of the huge reptiles that took place at that time. In the winter of 1854-5, four-fifths of the birds in Darwin’s grounds perished on account of the severity of the cold.