Isolation. 39 



regression in the average character of offspring to 

 the typical character of the group through reversion 

 or atavism (Natural Inheritance, p. 97) must have 

 the effect of neutralizing the segregative influence of 

 mere apogamy. That such, however, cannot be the 

 case has been well shown by Mr. Gulick in his paper 

 on Intensive Segregation. Without at all disputing 

 the validity of Mr. Galton's law, he proves that " it can 

 hold in full force only where there is free crossing, 

 otherwise no divergent race could ever be formed by 

 any amount of selection and independent breeding 1 ." 

 This is so self-evident that I need not quote his demon- 

 stration of the point. 



In conclusion, then, and having regard to the 

 principle of isolation as a whole, or in all the many and 

 varied forms in which this principle obtains, I trust that 

 I have redeemed the promise with which I set out 

 viz. to show that in relation to the theory of descent 

 this principle is of an importance second to no other, 

 not even excepting heredity, variability, and the 

 struggle for existence. This has now been fully 

 shown, inasmuch as we have clearly seen that the im- 

 portance of the struggle for existence, and consequent 

 survival of the fittest, arises just because survival 

 of the fittest is a form, and a very stringent form, of 

 isolation ; while, as regards both heredity and vari- 

 ability, we are now in a position to see that the more 

 fully we recognize their supreme importance as 

 principles concerned in organic evolution, the more 

 must we also recognize that any rational theory of 

 such evolution becomes, in the last resort, a theory 



1 Zool. Journal Lin. Soc., vol. xxiii. p. 313. 



