i 4 2 PHYSICAL BASIS OF CIVILIZATION 



to eating the normal type. This makes great in- 

 roads upon the surviving numbers of the normal, but 

 permits the wasp-like to multiply. After some 

 generations, these conditions thin out the normal 

 type and increase the wasp-like to such an extent 

 that the creatures feeding on this species of flies 

 are tempted to emigrate to regions where the 

 normal type still predominates. This gives a new 

 chance of survival and multiplication to the nor- 

 mal type in that locality from which these enemies 

 have emigrated. Under the operation of the law 

 that the older hereditary tendencies in an organism 

 have far more power of resistance than the later 

 variations, and that reversions to the older type 

 increase whenever the conditions which have 

 favored new varieties begin to diminish in force; 

 the normal type in the last-named locality, then, 

 tends to increase proportionally much more in num- 

 bers than the wasp-like. After some generations 

 this causes the former again to predominate, which 

 is likely to tempt its enemies to return to that 

 locality, when the reverse of the process will com- 

 mence again and soon. See footnote, page 39, and 

 Appendix Note IV. 



When one type begins to diminish, it becomes 

 harder for both sexes of that kind to find mates 

 of their own variety, and this difficulty increases 

 the rapidity with which the other type tends to 

 predominate. 



Would not careful consideration of the process 



