340 GEOEGE JOHN EOMANES 1894 



94 St. Aldate's, Oxford (immediately opposite Christ 

 Church). I cannot talk long at a time, but I think 

 the meeting will be of use to both. 



Of course ' lsola.tion produces segregation of type,' 

 is only a short-hand expression, meaning — indis- 

 criminate variation being supposed — isolation supplies 

 a necessary condition to segregation of type by up- 

 setting the previous stability that was due to free 

 inter-crossing. 



I quite agree that Darwin very greatly over- 

 estimated the benefit of inter-crossing, as I am 

 showing in my forthcoming book on ' Physiological 

 Selection.' But this is quite a different thing from 

 his having made too much of inter-crossing as a con- 

 dition to stability of type ; I do not think that this 

 can be made too much of. Indeed, how is it con- 

 ceivable that there ever can be divergence of type 

 without isolation of some kind having first occurred 

 at the origin, and throughout the growth of every 

 branch? Moreover, I agree with you about self- 

 fertihsation, but see in it a form of physiological 

 selection ; it is one kind of sexual isolation, or 

 prevention of inter-crossing with neighbouring in- 

 dividuals. So that the more perfectly it obtains in 

 any given type, the better chance there is for that 

 type to become a new species by independent vari- 

 ability — and this whether or not the indep)endent 

 variability is likewise indiscriminate (or in your 

 terminology ' indefinite '). 



In my last letter I referred to the works of Jordan 

 and Nageli for any number of ^ facts in Nature of 

 varieties arising among the type forms.' I will show 



