THE EFFECT OF MAN 547 



Kirghiz, and in Germany chooses barns for its nesting site, as does 

 H. erythrog aster in America. The European house swallow (Delichon 

 urbica) , however, still nests in rocky situations in Spain. 17 



In North America the chimney swift has moved into chimneys 

 from the hollow trees formerly occupied and may occur in flocks 

 numbering thousands. The cliff swallow has moved from its cliff nest- 

 ing sites to the more abundant artificial cliffs furnished by man. and 

 other American swallows exhibit various stages of transition from 

 the use of natural nesting sites to the adoption of artificial ones. 

 Night hawks now nest by thousands on the flat roofs of city apart- 

 ment houses in place of on dry ground as in the past. Phoebes nest in 

 barns and under bridges, and of the mammals, bats fly into the arti- 

 ficial caves furnished by belfries or attics. 



Finally, there is a whole group of commensals and parasites di- 

 rectly associated with man. Among these are the silverfish (Lepisma), 

 cockroach (Periplaneta) , house cricket (Acheta domestica) , bedbug 

 (Cimex lectulcrius) , clothes moth (Tincola biselliella) , house fly 

 (Musca domestica), house mouse (Mus mus cuius) , and house rat 

 [Rattus norvegicus) . These forms are dependent upon human culture, 

 and are not to be found, for example, in ruins. They have followed 

 man around the earth and become cosmopolitan; where man does not 

 live, neither do they. They are accompanied by a host of less closely 

 associated forms which vary with climate, vegetation, and surround- 

 ing animal life. 



Tropical regions. — The conditions described previously in this 

 chapter are based primarily on the North Temperate Zone. The broad 

 outlines of the story of the effect of man's activities upon animal dis- 

 tribution are essentially the same for the tropics, with the exception 

 that both climatic and biotic conditions tend to make his alterations 

 of nature more difficult to accomplish and quicker to disappear once 

 his efforts relax. The depths of the primeval tropical forests are rela- 

 tively less populous than the margins. Man's activities extend the 

 forest margin conditions and create more sorts of niches which allow 

 the forest margin abundance of animal life to become extended. Man's 

 activities in planting tropical fruits allow an extension of fruit-eating 

 birds and bats, and even the gorilla has been described to be on the 

 increase in parts of mid-Africa where fruit growing has been at- 

 tempted. 



The clearing of forests and the introduction of fence or telephone 

 posts and wooden buildings have created new niches for the abundant 

 wood-eating termites of the tropical regions. Houses have furnished 

 new habitats for various tropical animals, among which, in addition 



