DISTRIBUTION. 



THERE are two questions to be answered in studying the 

 distribution of plants and animals : What in the past caused 

 them to be so scattered, and what in the present holds them 

 where they are ? 



The first could not be determined without the help of 

 the geologist, but the second flows naturally out of what we 

 have been studying. 



Birds are like people. Some need and enjoy much greater 

 heat than most others, and some few cannot live in regions even 

 moderately warm, but all of them desire and seek a place just 

 warm enough for their own constitutions. Furthermore, no 

 bird, however hardy he may be, can exist where his food will 

 be destroyed by cold or will be buried under snow and ice for 

 many months in the year. So choice and necessity, acting 

 together, drive the birds back and forth as the cold and the 

 food supply increase and diminish. For many months in 

 the year the bird is homeless, but as it comes summer he 

 always seeks some spot that promises just the right degree 

 of warmth and food enough for himself and family. The 

 breeding grounds are always reckoned as the home of the 

 bird, and maps showing distribution are supposed to show 

 us where the birds are found in the height of summer. 



Why is it then that birds whose natural home is in the 

 North leave behind them such lagging rear columns in lands 

 of sunshine and almost tropical heat? Why are the three- 

 toed woodpeckers, which are found elsewhere only in the 

 most northern of the Northern states, found also in a narrow, 



157 



