ONG 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, ora 
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 faune. Anisland 
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 
