128 ZOOLOGY. 



of an animal is not adapted to the life it is born into, it 

 must die. 



4. Increase or Stability of Numbers, Every wild 

 rabbit who lives to old age has probably been concerned 

 during its life as one of two partners in the production of 

 not less than threel hundred young rabbits. In Australia, 

 where rabbits have been introduced by man, and where 

 they have found abundance of food and an absence of 

 carnivora to feed on them, the result of this amazing 

 fertility is a tremendous increase in the number of rabbits. 

 But in England, where rabbits have long been established, 

 the case is different : there is little or no room for an 

 increase in numbers, and as a matter of fact we know that 

 there is no perceptible increase. We must therefore con- 

 clude that out of three hundred rabbits born, only two 

 survive to middle age, on the average : the rest being either 

 killed and eaten by carnivorous animals, or (more rarely) 

 dying through inability, for some reason or another, to 

 obtain food. 



5. Natural Selection. Every day of its life a wild 

 rabbit has to obtain food and escape being eaten, and failure 

 to do either the one or the other means certain death. In 

 a herbivorous species like the rabbit the obtaining of food 

 is an easy matter under all ordinary circumstances, and the 

 escaping of enemies is the great necessity of life. In the 

 case of a carnivorous animal the relative importance of 

 these two conditions is reversed. 



Now the ability of a rabbit to escape being eaten 

 depends, among other things, on its alertness in recognizing 

 an enemy's presence and its swiftness and sureness in 

 running ; and though chance plays a large part in deter- 

 mining out of several rabbits disturbed by a dog which 

 shall escape and which shall be eaten, still, if we consider 

 the result, not of one such event, but of several hundreds, 

 we may feel sure that the one rabbit out of one hundred 

 and fifty which repeatedly escapes to a good age must have 

 been more alert and a better runner than the average of 

 the one hundred and forty-nine. He may not have been 



