is gone, and the final step of extinction may often pass unnoticed.

But a few years ago the air in the Ohio Valley was dark in the season of migration with the hordes of passenger pigeons. The advance of a tree-destroying, pigeon-shooting civilization has gone steadily on, and now the bird which once crowded our Western forests is in the same region an ornithological curiosity. The extinction of the American bison or "buffalo," and the growing rarity of the grizzly bear, the wolf, and of large carnivora generally, furnishes cases in point. When Bering and Steller landed on the Commander Islands in 1741, the sea-cow, a large herbivorous creature of the shores, was abundant there. In about fifty years the species, being used for food by fishermen, entirely disappeared. In most cases, however, a species that crosses its limiting barriers, but is unable to maintain itself, leaves no record of the occurrence. We know, as a matter of fact, that stray individuals are very often found outside the usual limit of a species. A tropical bird may be found in New Jersey, a tropical fish on Cape Cod, or a bird from Europe on the shores of Maine. Of course, hundreds of other cases of this sort must escape notice; but, for one reason or another, the great majority of these waifs are unable to gain a new foothold. For this reason, outside of the disturbances created by man, the geographical distribution of species changes but little from century to century; and yet, when we study the facts more closely, evidences of change appear everywhere.

One class

152. Species altered by adaptation to new conditions. Of the third class or species altered in a new environment examples are numerous, but in most cases the causes involved can only be inferred from their effects. of illustrations may be taken from island fauna. is set off from the mainland by barriers which species of land animals can very rarely cross. On an island a few waifs of wave and storm may maintain themselves, increasing in

An island

FIG. 168-The manatee, or sea-cow (Trichechus latirostris). A living species of sescow related to the now extinct Steller's sea-cow.

FIG. 169. On the shore of Narborough Island, one of the Galapagos Islands, Pacific Ocean, showing peculiar species of sea-lions, lizards, and cormorants. Drawn from a photograph made by Messrs. SNODGRASS and HELLER.

numbers so as to occupy the territory; but in so doing only those will survive that can fit themselves to the new conditions. Through this process a new species will be formed, like the parent species in general structure, but having gained new traits adjusted to the new environment.

The Galapagos Islands are a cluster of volcanic rocks lying in the open sea about six hundred miles to the west of Ecuador. On these islands is a peculiar land fauna, derived from South American stock, but mostly different in species. Darwin noted there "twenty-six land birds; of these, twenty-one, or perhaps twenty-three, are ranked as distinct species. Yet the close affinity of most of these birds to American species is manifest in every character, in their habits, gestures, and tones of voice."

Among land animals similar migrations may occur, giving rise, through the adaptation to new conditions, to new species. The separation of species of animals isolated in river basins or lakes often permits the acquisition of new characters, which is the formation of distinct species in similar fashion. On the west side of Mount Whitney, the highest mountain in the Sierra Nevada of California, there is a little stream called Volcano Creek. In this brook is a distinct species or form of trout, locally called golden trout. It is unusually small, very brilliantly colored, its fins being bright golden, and its tiny scales scarcely overlap each other along its sides. This stream flows over a high waterfall (Agua Bonita) into the Kern River. The Kern River is full of trout, of a kind (Salmo gilberti) to which the golden trout is most closely allied. There can not be much doubt that the latter is descended from the former. With this assumption, it is easy to suppose that once the waterfall did not exist, or that through some agency we can not now identify certain fishes had been carried over it. Once above it, they can not now return, nor can they mix with the common stock of the river. Those best adapted to the little stream have survived. The process of adaptation has gone on till at last a distinct species (or sub-species *) is formed. In recent times the

* In descriptive works the name species is applied to a form when the process of adaptation seems complete. When it is incomplete, or

FIG. 170.-Locusts (Schistocerca) taken on the Galapagos Islands, Pacific Ocean; all descended from a common ancestor, but now scattered over the various islands, and varying in size and markings. a, Schistocerca melanocera Stol (Charles Island); b, S. intermedia borealis Snodgrass (Abingdon and Bindloe Islands); c, S. intermedia Snodgrass (Duncan Island); d, S. literosa Walker (Chatham Island); e. S. melanocera lineata Snodgrass (Albemarle Island); f, S. melanocera immaculata Snodgrass (Indefatigable Island). (The species intermedia is probably a hybrid between the other two species.)