Ch. XI.] XATUKAL AXD ARTIFICIAL SELECTION. 207 



This is, I think, in favour of the inference that the 

 variety has been produced by natural and not by artifi- 

 cial selection, for diminished fertility is seldom or never 

 acquired between artificial varieties. 



Man isolates varieties, and breeds from them, and 

 continuing to separate those that vary in the direction 

 he wishes to follow, a very great difference is, in a com- 

 paratively short time, produced. But these artificial 

 varieties, though often more different from each other 

 than some natural species, readily interbreed, and if left 

 to themselves, rapidly revert to a common type. In 

 natural selection there is a great and fundamental 

 difference. The varieties that arise can seldom be sepa- 

 rated from the parent form and from other varieties, 

 until they vary also in the elements of reproduction. 

 Thousands of varieties probably revert to the parent 

 type, but if at last one is produced that breeds only with 

 its own form, we can easily see how a new species might 

 be segregated. As long as varieties interbreed together 

 and with the parent form, it does not seem possible that 

 u, new species could be formed by natural selection, 

 excepting in cases of geographical isolation. All the 

 individuals might vary in some one direction, but they 

 could not split up into distinct species whilst they oc- 

 cupied the same area and interbred without difficulty. 

 Before a variety can become permanent, it must be 

 either separated from, the others, or have acquired some 

 disinclination or inability to interbreed with them. As 

 long as they interbreed together, the possible divergence 

 is kept within narrow limits, but whenever a variety 

 is produced, the individuals of which have a partiality 

 for interbreeding, and some amount of sterility when 



