Geographical Distribution 155 



rent itself. Even land animals may be carried over wide 

 stretches of water by winds and currents. The late Alfred 

 Russell Wallace records the transjjort of a boa constrictor 

 from Africa to St. Vincent, two hundred miles away, on a 

 floating cedar tree. Polar bears have occasionally been 

 stranded upon the shores of Iceland by icebergs. Lyell quotes 

 an early observer to the effect that "wolves, in the arctic re- 

 gions, often venture upon the ice near the shore, for the pur- 

 pose of preying upon young seals, which they surprise when 

 asleep. When these ice-floes get detached, the wolves are 

 often carried out to sea ; and though some may be drifted to 

 islands or continents, the greater part of them perish, and 

 have been heard in this situation howling dreadfully, as they 

 die by famine. According to the same authority travellers 

 in the Amazon country have on several occasions observed 

 monkeys, squirrels, crocodiles and other animals journeying 

 down that river on rafts of floating trees and tangled vege- 

 tation. Four pumas from such rafts are reported to have 

 visited ]\Iontevideo in one night."* 



Fresh water invertebrates may be carried by currents of 

 water or attached to the feet of birds, or by the wind in the 

 case of the eggs or cysts from the bottom of diy pools. 



JNIan's role in the spread of the animal inhabitants of the 

 earth is a very important one. The early Spanish explorers 

 brought horses to South America in 1537, "and the colony 

 being then for a time deserted, the horse ran wild ; in 1580, 

 only forty-three years afterwards, we hear of them at the 

 Strait of Magellan!"^ The spread of the English sparrow 

 and the brown rat, both introduced species from Europe, is 

 too well known to need repetition here, while the devastation 

 wrought among the trees of New England by the brown-tail 

 and the gypsy moths, is a warning example of the danger of 

 disturbing the scales which Nature ordinarily holds so nicely 

 balanced. 



The spread of animals and their occupation of the earth 

 resolves itself into a gi-eat obstacle race, and "the race is not 

 always to the swift." The barriers to the spread of animals 

 are manifold — temperature, moisture, sunlight, chemical char- 

 acter of water, mountains, rivers, lakes or seas, deserts, for- 

 ests and treeless plains are all barriers to various kinds of 

 animals. Temperature is probably the greatest obstacle to 

 the dispersal of marine animals. AVere it not for the trop- 

 ics intervening between the temperate and colder seas of 

 north and south their distribution would probably be world 



* Lyell, locus citatiis, p. 366. 



^Darwin, "Voyage of a Naturalist," p. 233. D. Appleton and 

 Company. 



