276 ANIMAL LIFE 



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 herbivo- 

 rous 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 main- 

 tain 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. 



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 in- 

 volved can only be inferred from their effects. One class 

 of illustrations may be taken from island faunae. An island 

 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 



