Chap. XXI. SELECTION. 235 



be a fair chance of variations of the desired nature arising, 

 and of every individual with the slightest blemish or in any 

 degree inferior being freely rejected. Hence length of time 

 is an important element of success. Thus, also, reproduction 

 at an early age and at short intervals favours the work. 

 Facility in pairing animals, or their inhabiting a confined 

 area, is advantageous as a check to free crossing. "Whenever 

 and wherever selection is not practised, distinct races are not 

 formed within the same country. When any one part of the 

 body or one quality is not attended to, it remains either un- 

 changed or varies in a fluctuating manner, whilst at the same 

 time other parts and other qualities may become permanently 

 and greatly modified. But from the tendency to reversion and 

 to continued variability, those parts or organs which are now 

 undergoing rapid improvement through selection, are likewise 

 found to vary much. Consequently highly-bred animals when 

 neglected soon degenerate ; but we have no reason to believe 

 that the effects of long- continued selection would, if the con- 

 ditions of life remained the same, be soon and completely lost. 

 Man always tends to go to an extreme point in the selection, 

 whether methodical or unconscious, of all useful and pleasing 

 qualities. This is an important principle, as it leads to con- 

 tinued divergence, and in some rare cases to convergence of 

 character. The possibility of continued divergence rests on 

 the tendency in each part or organ to go on varying in the 

 same manner in which it has already varied ; and that this 

 occurs, is proved by the steady and gradual improvement of 

 many animals and plants during lengthened periods. The 

 principle of divergence of character, combined with the 

 neglect and final extinction of all previous, less-valued, and 

 intermediate varieties, explains the amount of difference and 

 the distinctness of our several races. Although we may have 

 reached the utmost limit to which certain characters can be 

 modified, yet we are far from having reached, as we have 

 good reason to believe, the limit in the majority of cases. 

 Finally, from the difference between selection as carried on 

 by man and by nature, we can understand how it is that 

 domestic races often, though by no means always, differ in 

 general aspect from closely allied natural species. 



