DARWIN AND HIS CRITICS, 63 



IV. " The opinion that species have 

 definite though very different limits to 

 their variability, is still tenable." 

 This is fully admitted by Wallace,* who re- 

 marks, "In the matter of speed, a limit of a 

 definite kind as regards land animals does exist 

 in nature. All the swiftest animals, deer, hares, 

 antelopes, foxes, lions, leopards, horses, zebras, 

 and many others, have reached very nearly the 

 same degree of speed. Although the swiftest of 

 each must have been for ages preserved, and the 

 slowest must have perished, we have no reason 

 to believe there is any advance of speed. The 

 possible limit under existing circumstances, and 

 perhaps under possible terrestrial conditions, 

 has been long ago reached." Murphy, f how- 

 ever, suggests that the limits which undoubtedly 

 exist to unlimited variation in one direction, 

 may perhaps be due to the counteracting in- 

 fluence of a tendency to reversion ; and that if 

 we could keep our racehorses (say) at their 

 maximum of speed during a sufficient number 

 of generations, this tendency might in time be 



* " Natural Selection," pp. 291294. 



t " Habit and Intelligence," vol. i. p. 218. 



