34 PHYSICAL BASIS OF CIVILIZATION 



animals among individuals of the same race and 

 variety which has remained practically unaltered 

 since thousands of years, dating back even into the 

 prehistoric ages, that in this class of creatures the 

 changes which originate as above stated — and there 

 is no scientific evidence that among them such 

 alterations can originate or ever have originated in 

 any other way — must come into existence at first 

 by very small increments, which are then left to 

 increase from generation to generation, or else to 

 diminish until they disappear. 



Since, then, from the present state of the evidence 

 it must be admitted that new traits in such organ- 

 isms can only arise from small increments produced 

 by variation, therefore is it in this place unnecessary 

 to consider the mooted problems of periods of ex- 

 treme mutability, alternating with others of extreme 

 stability, which have lately been suggested in con- 

 nection with observations on toad-flax, evening 

 primroses, and other plants of comparatively low 

 organization. These problems will be carefully dis- 

 cussed later in this chapter, and on that occasion no 

 efforts will be spared to demonstrate as clearly to 

 the mind of the reader as it is to the mind of the 

 writer that the inferences drawn from these obser- 

 vations cannot rationally be applied to the more 

 highly specialized animal organisms, and therefore 

 not to this argument. 



When such new traits produce better adaptation, 

 then as a matter of course individuals affected by 





