Permanence and Evohttion, 19 



getting through a foot in a year ; but we should 

 think rather ill of a physician who, finding that 

 a patient who when he first came to him could 

 only walk two miles an hour, at the end of six 

 months could walk four miles, should presume 

 that at the end of a year his patient would be 

 equal to eight miles per hour, and at the end of 

 two years to sixteen miles. 



A living organism is more like a machine 

 than like anything else we know of, and we 

 know that machines can be played tricks with 

 in certain respects up to a certain degree, and 

 yet continue to go on working ; while in other 

 respects, and to more than a certain point in 

 any respect, they totally refuse to be interfered 

 with. It seems then that the principles we 

 ought to go upon in discussing evolution are 

 these : 



First, to find out by carefully recorded and 

 sifted experience what characters, if any, are 

 liable to variation, and in what respects, care 



