Chap. X.] THE LAPSE OF TIME. 57 



animals, which propagate their kind much more slowly 

 than most of the lower animals, that they have formed 

 what well deserves to be called a new sub-breed. Few men 

 have attended with due care to any one strain for more 

 than half a century, so that' a hundred years represents 

 the work of two breeders in succession. It is not to be 

 supposed that species in a state of nature ever change 

 so quickly as domestic animals under the guidance of 

 methodical selection. The comparison would be in 

 every way fairer with the effects which follow from 

 unconscious selection, that is the preservation of the 

 most useful or beautiful animals, with no intention of 

 modifying the breed ; but by this process of unconscious 

 selection, various breeds have been sensibly changed in 

 the course of two or three centuries. 



Species, however, probably change much more slowly, 

 and within the same country only a few change at the 

 same time. This slowness follows from all the inhabit- 

 ants of the same country being already so well adapted 

 to each other, that new places in the polity of nature do 

 not occur until after long intervals, due to the occur- 

 rence of physical changes of some kind, or through the 

 immigration of new forms. Moreover variations or 

 individual differences of the right nature, by which 

 some of the inhabitants might be better fitted to their 

 new places under the altered circumstances, would not 

 always occur at once. Unfortunately we have no means 

 of determining, according to the standard of years, how 

 long a period it takes to modify a species ; but to the 

 subject of time we must return. 



