274 RECAPITULATION. [Chap. XV. 



modified species, if we were to examine the two ever so 

 closely, unless we possessed most of the intermediate 

 links ; and owing to the imperfection of the geological 

 record, we have no just right to expect to find so many- 

 links. If two or three, or even more linking forms were 

 discovered, they would simply be ranked by many 

 naturalists as so many new species, more especially if 

 found in different geological sub-stages, let their dif- 

 ferences be ever so slight. Xumerous existing doubtful 

 forms could be named which are probably varieties ; 

 but who will pretend that in future ages so many fossil 

 links will be discovered, that naturalists will be able to 

 decide whether or not these doubtful forms ought to be 

 called varieties ? Only a small portion of the world 

 has been geologically explored. Only organic beings of 

 certain classes can be preserved in a fossil condition, at 

 least in any great number. Many species when once 

 formed never undergo any further change but become 

 extinct without leaving modified descendants ; and the 

 periods, during which species have undergone modifica- 

 tion, though long as measured by years, have probably 

 been short in comparison with the periods during winch 

 they retained the same form. It is the dominant and 

 widely ranging species which vary most frequently and 

 vary most, and varieties are often at first local — both 

 causes rendering the discovery of intermediate links in 

 any one formation less likely. Local varieties will 

 not spread into other and distant regions until they are 

 considerably modified and improved; and when they 

 have spread, and are discovered in a geological formation, 

 they appear as if suddenly created there, and will be 

 simply classed as new species. Most formations have 

 been intermittent in their accumulation; and their 



