iv] NUMBERS AND DENSITY OF SPECIES 75 



In thousands Numb. 



of of Areal 



square miles, species. density. Group of animals. 



Total habitable land 40,000 25,000 1600 All vertebrates 



,, ., ,, 4500 8888 Amphibia & reptiles 



,, 12,000 3333 Birds 



2500 16,000 Mammals 



,, ,, ,, 6000 6666 Freshwater fish 



British India 1678 1630 1000 Birds 



666 2520 Amphibia & reptiles 



Rich and poor are arbitrary conceptions. More- 

 over a country may be rich in birds and poor in some 

 other class. We may, however, fairly assume that, 

 class for class, an index below the world's average 

 means a decidedly poor fauna, and an index double 

 that of the world means a rich fauna. The reptilian 

 fauna of India is as notoriously rich as that of North 

 America is poor, and that of Europe is still poorer. 

 The bird faunas of South America and India are twice 

 or even three times as rich as those of Europe. Such 

 calculations yield valuable results, sometimes so start- 

 ling that the underlying causes have to be inquired 

 into, which then occasionally reveal unexpected cor- 

 relations. 



