232 EVOLUTION IN NATURE AND UNDER DOMESTICATION. 



nature must by the very nature of the conditons which make it 

 possible be a rather rare process, though not necessarily a slow 

 one when it occurs, new cultivated plants and animals origi- 

 nate frequently. 



The main point of difference is this, that we have good rea- 

 son to assume that whereas species in nature may be replaced 

 by others, but do not change under natural selection after 

 they are once formed, domestic species do change by selection. 



In most domestic animals and plants there is no pure, unse- 

 lected multitude into which varieties merge and which con- 

 stitutes the type of the species. Propagation under the favor- 

 able conditions of cultivation is so quick, that the whole mass of 

 individuals of a certain strain is commonly descended from 

 very few individuals a few generations back Under the hands 

 of a few breeders every sub-breed becomes a species and it chan- 

 ges rapidly. A few, a very few individuals who are more like the 

 pre-conceived ideal toward which the group is bred, are care- 

 fully bred, and it is seen to, that their progeny is as numerous 

 as possible so that the whole group varies in their direction. In 

 most domestic animals only a very small minority of the males 

 are used for breeding at all, and there selection of a few males has 

 a very great influence on the whole breed. In all the cultivated 

 animals and plants there is always enough cross-breeding to 

 keep up the potential variability necessary for a further change 

 under selection. Even in the more highly-bred animals oc- 

 casional animals are registered, and therefore taken up into 

 the groups which here are species, which have a more or less 

 remote ancestor belonging to some other species. 



In this connection it becomes necessary to reexamine the 

 proofs which Darwin adduced for the monophyletic origin of 

 some of our most variable domestic animals. For it stands to 

 reason that, if it is true, as it appeared to Darwin, that selec- 

 tion within one species can produce such different animals as a 

 Jacobin and a Fantail pigeon, or as a Silky and a Polish fowl, 

 selection in nature must influence species and modify them 

 continually. Darwin conceded the polyphyletic origin of the 



