4 o PHYSICAL BASIS OF CIVILIZATION 



and recent modifications which are very unfavor- 

 able to their survival? Only if the new traits 

 inexorably forced upon those affected, firstly, an 

 entire change of habitat, away and apart from 

 where their nearest relatives of the previous genera- 

 tion exist ; and secondly, if in addition they inexor- 

 ably enforced an equally absolute differentiation in 

 actions and habits related to the more important 

 requirements for sustaining life, — only then could 

 panmixia be prevented in the natural course of 

 events. And such radical changes in habitat and 

 modes of life seem unthinkable, if not impossible, 

 of occurrence, in a few brief generations, unless mod- 

 ifications have arisen which resemble in their rapid- 

 ity and violence, those sudden mutations which 

 have been reported to take place among the low- 

 liest forms of animal and vegetable life. As to 

 these, however, as already mentioned, it will be 

 proven later in this chapter, that among higher 

 animals they are impossible without rapidly fatal 

 results to those affected by them, and they are 

 therefore incompatible with survival. 



There remains, then, but one possibility in which 

 panmixia can be prevented in the natural course of 

 events among closely related varieties, and that is 

 when structurally slight modifications possess the 

 power of producing tremendously great and won- 

 derful consequences. This last phrase is exactly 

 descriptive of the unique phenomena which occurred 

 when the "Physical Basis of Civilization," referred 



