The Evolution of Organisms 159 



for the rapid extension and dominance of the varia- 

 tion if once it is favored by selective breeeding." 

 Modifications. Besides variations which spring 

 from within emerging from the penetralia of the 

 germ-cells, where lies the fountain of all lasting 

 organic change there are modifications superin- 

 duced from without. They may be defined as 

 changes wrought in the body of an individual dur- 

 ing its lifetime, as the direct result of changes in 

 function and environment, which so transcend the 

 limits of organic elasticity that they persist after 

 the inducing conditions have ceased to operate. 

 The peculiarities in our finger prints are variations, 

 but the callosities on our hands are modifications. 

 The inborn peculiarity of our facial physiognomy 

 is a variation, but sunburning which lasts for years 

 is a modification. These modifications or ac- 

 quired characters are often of great personal im- 

 portance and they may also serve as temporary 

 shields or screens for incipient inborn variations 

 in the same direction, but they have not been 

 proved to be of direct importance in the evolution 

 of races, since there is no convincing evidence that 

 they can be transmitted as such or in any represen- 

 tative degreee. In short, organic progress is 

 primarily due to changes in heritable Nature, not 

 to changes in Nurture. 1 



iSee J. Arthur Thomson, "Heredity " Murray, London, 

 1908. 



