iv] NUMBERS AND DENSITY OF SPECIES 73 



Pliocene, the rate of increase necessary to account for 

 the present total population would be so small as 

 to render the calculation quite preposterous. The 

 necessary gain would be 1000 : 1002 per generation ; 

 2 per 1000 people every 30 years, instead of 14 per 

 1000 annually as is now the case in England. This 

 implies that of all the thousands of children born to 

 500 couples during 30 years only two survived to add 

 to the race. This being an unthinkable state of 

 affairs, it follows that the rate of propagation has 

 been always as large as it is now all over the world, 

 but over and over again whole tribes have been well- 

 nigh wiped out, by each other, by epidemics or by 

 accidents. Just like the Red Indians of North 

 America, who, instead of well filling that large con- 

 tinent, have reasonably been computed never to have 

 amounted to more than a few hundred thousand 

 souls. 



What constitutes a rich or a poor fauna? The 

 number of species and the number of the greater 

 groups, or variety of types, to which the species 

 belong. For instance, the mammalian fauna of Aus- 

 tralia cannot be called rich, since it consists mainly of 

 the variations of one type only, that of marsupials. 



The tropical belt of the world is teeming with 

 a great variety of terrestrial life, while the number 

 of species decreases to almost nought towards the 

 poles. The arctic and antarctic regions are so poor 



