=Species-extinguishing and species-forming.=--Accompanying the constant slow migrating of species into new habitats and the constant slow changing of environment and conditions everywhere is to be seen a constant modification of the fauna of any region due to the inability of some species to maintain their ground, the predominating increase of others, and the modifying or adaptive changing of others into new forms. In 1544 the black rat of Europe was introduced into America and it soon crowded out the native rats, being in its turn crowded out by the European brown rat (the present common rat in buildings), introduced about 1775. Here we have the original native species unable to maintain itself in competition with introduced forms. With a change of environing conditions, certain species are unable to maintain themselves. With the destruction of the forests going on in parts of our country the great host of wood-creatures, the bears, squirrels, the wood-birds and insects, can no longer maintain themselves, and grow rare and disappear. Man often also influences the status of a species by checking its increase either by actual slaughter, as with the bison and passenger-pigeon, or by making adverse changes in its environment, as by destroying forests, or putting the plains under cultivation. In the discussion of "species-forming" (see p. 408) it was shown that adaptation may lead to the altering of species, and to the formation of new ones (under the influence of natural selection). With the gradual change of conditions, or with the facing of new conditions because of an unusual migration to or invasion of new territory, those individuals of the species exposed to the new conditions must adapt themselves in structure and habit in order to meet successfully the new demands. By the cumulative action of natural selection these adaptive changes are emphasized; and this emphasis may come to be so pronounced that the part of the species represented in this newly acquired territory, if isolated from the original stock, is so altered as to be quite distinct in appearance from the old. If these changed individuals are also physiologically distinct from the old stock, i.e. are unable to mate with them, a new species is established. As already mentioned, the peopling of islands from mainlands is an excellent and readily observable example of the phenomena referred to in the third law of distribution. FOOTNOTE: [20] Jordan and Kellogg's "Animal Life," 1900, p. 274. APPENDICES EQUIPMENT AND METHODS APPENDIX I EQUIPMENT AND NOTES OF PUPILS