202 GEOEGE JOHN EOMANES 1881- 



of allied species which grow intermixed in common 

 areas, proving by experiment that in all the cases 

 there is some comparative degree of sterility between 

 them (if only due to pre-potency of their own pollen), 

 would you regard this as making in favour of physio- 

 logical selection ? Or are you already prepared to 

 admit that such 7nust be the case, since otherwise 

 the species A and B could not exist without fusion 

 into one ? If you say that you are prepared to admit 

 this, it seems to me that you have already accepted 

 the theory of physiological selection on a priori 

 grounds. 



Again, if I should pubHsh one hundred other in- 

 stances of allied species topographically isolated from 

 one another, all of which were proved by experiment 

 to present no degree at all of mutual infertihty (so 

 that A X A and B x B are not more fertile than 

 crosswise), would you allow that, taken in conjunc- 

 tion with the previous set of experiments, these 

 finally prove the theory of physiological selection to 

 be true ? If not, I do not see how it is possible to 

 verify the theory at all : it is only by means of these 

 two complementary lines of research that, as it seems 

 to me, the theory can be experimentally tested. 



In the former case — i.e. where alhed species 

 intermix in common areas — sometimes they inter- 

 cross freely (e.g. Primula vulgaris and veris^ Geuvi 

 urbanum and rivale, Bumex, Epilobium, &c.), while 

 in other instances they don't (e.g. Banunculus 

 repens and bulbosus, Lepidium Smithii and campestre, 

 Scrophularia nodosa and aquatica, &c.). Now, as 

 regards the latter, I suppose you would not question 



