Permanence and Evolution. 61 



and produce perfectly fertile offspring, which is 

 rarely the case with wild species. 



6th. All the domestic races, while exhibiting 

 isolated, odd characters, which are almost of 

 generic value, in all other, or nearly all other 

 points, resemble each other and the rock dove. 

 Darwin also mentions that in all or most breeds 

 there occur birds with the peculiar markings of 

 the rock dove. 



As to these points, I have to remark that 

 some of them seem to me of little importance 

 in any case, while others might undoubtedly 

 have their weight, if it were otherwise known that 

 modification, and such particular direction of 

 modification as is required, was a vera caiisa, while 

 they are totally incompetent to prove it. As to 

 the first consideration, Darwin would seem entirely 

 to have forgotten all he ever wrote about our 

 ignorance of the causes of extinction, when he 

 assumes that wild birds, which have become 

 extinct since the historical period, have there- 



