12 ORGANJC EVOLUTION 



Adults Young 



Fourth year ...... 54 108 



Fifth year ...... 162 324 



Sixth year ...... 486 972 



Seventh year ..... I >458 2,916 



Eighth year ...... 4-374 8,748 



Ninth year ...... 13,122 26,244 



Tenth year . . 39,3 6 6 7 8 >73 2 



End of tenth year .... 118,098 



End of twentieth year .... 20,913,948,846 



We see at once that the earth could not support the 

 animals of even a single species that would arise were not 

 the natural increase of the species held in check. 



As a matter of fact, the number of animals or plants of 

 any given species remains about constant. There are usu- 

 ally no great fluctuations from year to year. To return, 

 then, to our illustration of the robin, we can say that more 

 birds (including eggs and young) die every year than live. 

 If the whole number remains constant from year to year 

 and if each pair of robins have four young yearly, of 

 course four robins die every year for each two that sur- 

 vive. That is, the death-rate is twice as great as the total 

 permanent population. 



This death-rate is greatly surpassed by that of many 

 species both of animals and plants, which have a much 

 larger yearly birth-rate. Among mammals the average 

 birth-rate would perhaps be no greater than it is among 

 the robins, but among birds are many which have twice 

 or three times or even four times as many young each 

 year as do the robins; e.g. the whole grouse tribe, includ- 

 ing the pheasants, the partridges, and the quail, also the 

 wild jungle-fowl from which our domestic chickens have 

 been derived. Snakes, turtles, lizards, and most reptiles 



