74 THE WANDERINGS OF ANIMALS [CH. 



in permanently resident species, and devoid of Am- 

 phibia, reptiles and freshwater fishes, that they may 

 be left out of a census of the world's ' terrestrial in- 

 habitants ' if we want to compare the various land 

 complexes as to the richness of their faunas. To 

 illustrate shortly the principle involved, we must 

 further restrict ourselves to the number of species. 

 The bulk of generally habitable land may be reduced 

 to the conveniently round number of 40 million square 

 miles. 



The vertebrate fauna, omitting strictly marine 

 fishes, may be stated in the following conservative 

 numbers of species : freshwater fishes, 6000 ; Am- 

 phibia, 1000 ; reptiles, 3500 ; mammals, 2500 ; birds, 

 12,000. Total, 25,000 species. 



This gives on an average one species to every 

 1600 square miles, and this may be called the areal 

 density of the world's vertebrates. 



The areal density is expressed by the quotient of 

 the number of species of a country into the area in 

 square miles. The quotient, therefore, expresses the 

 specific areal density in an inverse ratio. The richer 

 the country, or the greater its number of species, the 

 smaller is the index figure. Examples : 



