170 Darwin, and after Darwin. 



sterility arises, there the condition is given for a new crop of 

 departures (species of a genus) ; and when genera are formed 

 by the occurrence of this bar, there natural selection and all 

 other equilibrating causes are supplied with new material for 

 carrying on adaptational changes in new directions. Thus, 

 owing to cross-infertility, all these causes are enabled to 

 work out numberless adaptations in many directions (i. e. 

 lines of descent) simultaneously. 



Cross-infertility and Stability. The importance of sterility 

 as a diagnostic feature is obvious if we consider that more 

 than any other feature it serves to give stability to the type ; 

 and unless a type is stable or constant, it cannot be ranked 

 as a species. That Darwin himself attributes the highest 

 importance to this feature as diagnostic, see Forms of Flowers, 

 pp. 58, 64. 



Cross-infertility and Specific Differentiation. In their 

 elaborate work on the many species of the genus Hieracium, 

 Nageli and Peter are led to the general conclusion that the 

 best defined species are always those which display absolute 

 sterility inter se\ while the species which present most 

 difficulty to the systematist are always those which most 

 easily hybridize. Moreover, they find, as another general 

 rule applicable to the whole genus, that there is a constant 

 correlation between inability to hybridize and absence of 

 intermediate varieties, and, conversely, between ability to 

 hybridize and the presence of such varieties. 



Cross-infertility in Domesticated Cattle. Mr. J. W. Crompton, 

 who has had a large experience as a professional cattle- 

 breeder, writes to me (March 2, 1887) 



"That form of barrenness, very common in some districts, 

 which makes heifers become what are called 'bullers' that 

 is, irregularly in 'season,' wild, and failing to conceive is 

 certainly produced by excess of iron in their drinking-water, 

 and I suspect also by a deficiency of potash in the soil." 



