320 Habit and Instinct. 



8. Suppose, however, that a group of plastic organ- 

 isms is placed under new conditions. 



9. Those whose innate plasticity is equal to the occa- 

 sion are modified, and survive. Those whose plasticity is 

 not equal to the occasion are eliminated. 



10. Such modification takes place generation after 

 generation, but, as such, is not inherited. There is no 

 transmission of the effects of modification to the germinal 

 substance. 



11. But variations in the same direction as the modi- 

 fications, are now no longer repressed and are allowed full 

 scope. 



12. Any congenital variations antagonistic in direction 

 to these modifications will tend to thwart them and to 

 render the organism in which they occur liable to 

 elimination. 



13. Any congenital variations similar in direction to 

 these modifications will tend to support them and to favour 

 the organism in which they occur. 



14. Thus will arise a congenital predisposition to the 

 modifications in question. 



15. The longer this process continues, the more marked 

 will be the predisposition, and the greater the tendency of 

 the congenital variations to conform in all respects to the 

 persistent plastic modifications ; while 



16. The plasticity still continuing, the modifications 

 become yet further adaptive. 



17. Thus plastic modification leads, and germinal 

 variation follows : the one paves the way for the other. 



18. Natural selection will tend to foster variability in 

 given advantageous lines when once initiated ; for (a) the 

 constant elimination of variations leads to the survival 

 of the relatively invariable; but (b) the perpetuation of 

 variations in any given direction leads to the survival of 



