THE STRUGGLE TO LIVE 409 



the Atlantic Ocean to America. In fact the common black 

 rats and brown rats- of the houses and barns over this whole 

 country are not native rats at all, but are descendants of 

 European rats unintentionally brought across the ocean in 

 ships. The same is true of many of the insect pests which 

 trouble us; for example, the Hessian fly, which does great 

 damage to wheat, the cockroaches of our houses and the 

 carpet beetles or buffalo bugs which attack rugs and carpets. 

 Sometimes a boring insect lying snugly in a log gets carried 

 down a river, out into the ocean, and by means of ocean 

 currents far away to some island where it may crawl out 

 and lay eggs and so establish itself in a new country. Some- 

 times animals are intentionally imported by man from 

 foreign countries. The introduction of the English sparrow 

 into this country and the rabbits into Australia are examples 

 of unfortunate experiments along this line. 



Map showing the distribution of animals. — Zoolo- 

 gists have been studying the distribution of animals so long 

 that they have been able to map out the range of many of 

 the well-known kinds. On a map of the world they in- 

 dicate, by shading, all those regions in which lions exist; 

 all those in which elephants live, and all those in which 

 humming-birds are found. Now this kind of map-making 

 reveals many things of interest and throws much light on 

 the relations of animals to climate, to geography, and to 

 each other. 



Such zoological map-making may be restricted to a limited 

 locality, and is the best way for beginning students to study 

 distribution. On a large sheet of strong paper a map of 

 the region, say one or two miles square, about the school- 

 house, should be made, with all the streams, ponds, swamps, 

 pastures, woods, etc. Then search carefully for the haunts 

 of certain kinds of animals which are known to occur in 

 the mapped region, and mark them on the map. It will 

 soon be found that the different kinds of animals are more or 



