174 GEOEGB JOHN EOMANBS issi- 



•demonstrably sterile with the original pa,rent form ? 

 Therefore, what I see in these facts is precisely what, 

 upon my theory, I should expect to see, viz. first, a 

 ' primary variation,' or ' physiological sport,' arising 

 at long intervals ; secondly, closely following upon 

 this, a crop of ' secondary variations ' in the way of 

 -slight morphological changes affecting two or three 

 different ' strains ' simultaneously ; and thirdly, an 

 •eventual blending of these strains by intercrossing 

 with one another without being able to intercross 

 with the surrounding and (at first) very much more 

 numerous parent form. 



But I can now quite understand why you thought 

 these facts were ' dead against ' me ; you thought 

 that every single sHght change of morphology must 

 (on my theory) have had a separate ' physiological 

 sport ' to account for it. This, however, most em- 

 phatically is not my theory. Physiological isola- 

 tion I regard as having morphological consequences 

 precisely analogous to those of geographical isolation ; 

 and you would not think of arguing that there must 

 be a separate geographical isolation for every slight 

 change of structure — for example, that a pecuhar 

 species of plant growing on a mountain top must 

 have had one isolation to explain its change of 

 form, and another isolation to explain its change of 

 colour. 



Lastly, if you will look up Hilgendorf's paper 

 about these snails of Steinheim, I think you will find 

 it impossible to suppose that all these little changes 

 (thus arising at long intervals in crops) can have 

 been useful. Or, if you can still doubt, look up the 



