THE RABr.IT. 129 



the very best-adapted of all to the necessities of a rabbit's 

 life, bat he was certainly not far short of the best. Now, 

 what constitutes better or worse adaptation? Precisely 

 those details of structure in which variation occurs. If, for 

 example, the possession of a thirteenth pair of ribs enables 

 a rabbit to breathe better during a run for life, or if a 

 jugular anastomosis facilitates the proper distribution of 

 blood to the organs that need it in an emergency, then we 

 may be sure that these two features will be commoner among 

 the rabbits that live long lives than among those that do 

 not.* And since it is the longest-lived rabbits that are 

 parents to the largest portion of the next generation, those 

 features will be commoner in the next generation than in 

 the present one. 



If these same conditions determining life and death are 

 repeated from generation to generation and generations 

 overlap and succeed one another rapidly in the rabbit 

 then those features which are of the nature of better adapta- 

 tions to external conditions will become commoner and 

 commoner in successive generations, until at last there may 

 be no rabbit without them, while those whose absence 

 means better adaptation will become rarer and rarer, until 

 no rabbit will have them. 



Thus the general structure of animals (for what has 

 been said of the rabbit might be said, mutatis mutandis, of 

 any other species) must be continually becoming more and 

 more perfectly adapted to their habitat and mode of life, and 

 the special opportunities and dangers which these present. 

 It is convenient to have a name for the process by wlr'ch 

 this adaptation is brought about ; and since it is essentially 

 a process of selecting out of the many individuals born the 

 few that shall live to become parents, and since this selecting 

 is done unconsciously by the multitude of external agencies 

 which we collectively call nature (including all other organ- 

 isms than the one under consideration), the term NATURAL 

 SELECTION is appropriately given to this process. 



* It is not asserted that such is actually the case : it is onlj 

 nssumed for purposes of illustration. If the reverse is the case, then 

 those features will be less common in the long-lived rabbits. 



ZOOL, 9 



