IMPERFECTION OF GEOLOGICAL RECORD 63 



formed wliat 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 guid- 

 ance 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 modify- 

 ing the breed; but by this process of unconscious selec- 

 tion 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 inhabi- 

 tants 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 occurrence 

 of physical changes of some kind, or through the immi- 

 gration of new forms. Moreover, variations or individual 

 differences of the right nature, by which some of the in- 

 habitants 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. 



On the Poorness of Paleoniological Collections 



Now let us turn to our richest geological museums, 

 and what a paltry display we behold! That our collec- 



— SCIEXCE — 20 



