43 8 ELEMENTARY ZOOLOGY 



for example, are carried by their hosts in all their travels ; 

 the tiny Tardigrada and Rotifera, which can be desiccated 

 and yet restored to active life by coming again into water, 

 are carried in the dried mud on the feet of birds or other 

 animals. On floating objects in rivers or in ocean cur- 

 rents land-animals may be carried long distances. Man, 

 the greatest traveller of all, is responsible for the widened 

 distribution of many animals. Thus have come to us in 

 ships from Europe the black and brown rats, the English 

 sparrow, the Hessian fly, the commonest cockroaches of 

 our houses and many other forms. And these animals 

 have been carried involuntarily all over the United States 

 in railway-cars and wagons. 



Barriers to distribution. As is indicated in the para- 

 graph on the modes of migration, a considerable sheet of 

 water is obviously a barrier to the further travelling of a 

 walking or crawling land-animal, although no barrier to 

 a winged form. Similarly a strip of land is a barrier to a 

 strictly aquatic animal as a fish. Or a high fall in the 

 stream may serve as an insuperable barrier, making it im- 

 possible for any fish below the fall to reach the upper 

 part of the stream. Numerous cases of this kind are 

 known in the Rocky Mountains and Sierra Nevada, where 

 a stream may be well supplied with trout below a fall, 

 and quite bare of these fish above the fall. In the 

 Yosemite Valley in California trout live in the Merced 

 River below the great Vernal and Nevada falls, but above 

 these falls the Merced contains no trout. To fresh-water 

 swimming animals salt water may be a barrier; thus some 

 kinds of fresh- water fishes are limited to one of two 

 near-by streams although the mouths of these streams 

 empty near each other into the ocean. The amphibious 

 batrachians, at home in fresh water and on land, are 

 killed by contact with sea-water. Earthworms also are 

 killed by salt water. Thus the narrowest ocean strait is 



