so AN INTRODUCTION TO ZOOLOGY. 



organisms. That interpretation lias been accepted 

 by every zoologist ^ as tlie key wliicli shows order and 

 beauty in what were previously the almost chaotic 

 details of natural history. It may briefly be shimmed 

 in the following words. 



Every plant and animal under favourable circum- 

 stances produces, to succeed the two parents, a 

 plurality of offspring, some kinds an immense number 

 of offspring.^ The result is, a " struggle for existence " 

 among the numerous offspring, each striving with, the 

 others for place and food. In this strife, the slightest 

 circumstance in which any individual differs from the 

 others will turn to its advantage or disadvantage, as 

 the case may be. Now, no two individuals are quite 

 alike, and no offspring precisely resembles its parent ; 

 to this tendency of individuals to present individual 

 peculiarities not derived from the parent, Darwin gave 



^ Two or three names might be given as exceptions, but it 

 would be difficult to find a fourth. 



" " Every organic being," Darwin wrote, " naturally in- 

 creases at so high a rate that, if (they were) not destroyed, 

 the earth would soon be covered by the progeny of a single 

 pair. . . . The elephant is reckoned the slowest breeder 

 of all known aniilials, and I have taken some pains to estimate- 

 its probable minimum rate of natural increase ; it will be 

 safest to assume that it begins breeding when thirtj- years 

 old, and goes on breeding till ninety years old, bringing forth 

 six young in the interval, and surviving till one hundred years 

 old ; if this be so, after a period of from 740 to 7-50 years there 

 would be nearly nineteen million elephants alive, descended 

 from the first pair." — Origin of Species. 



