88 Permanence and Evolution. 



research inverted, and Pelion piled upon Ossa 

 by using first the smallness of the difference 

 between domestic varieties to support a guess 

 that they descended from one pair, and then 

 making that last support a still wilder guess as 

 to the common origin of genera, orders, and 

 classes. 



To take a very small instance, the presence 

 of callosities on all four legs of the horse 

 (as distinguished from the other equi), how 

 can we have reason to suppose that that has 

 been evolved ; and granting that all the 

 varieties and breeds of domestic horse have one 

 origin, what variation do they offer at all analo- 

 gous to that ? The only answer I have ever 

 seen made to this is, that the differences existing 

 between natural species are produced by natural 

 selection, which can act on all parts of the 

 system indifferently ; whereas the differences 

 between domestic breeds are produced by man's 

 selection, which can only single out externally 



