274 RECAPITULATION. [Chap. XV. 



fied species, if we were to examine the two ever so close- 

 ly, 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 dis- 

 covered, they would simply be ranked by many natu- 

 ralists as so many new species, more especially if found 

 in different geological sub-stages, let their differences 

 be ever so slight. Numerous 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 modification, 

 though long as measured by years, have probably been 

 short in comparison with the periods during which 

 they retain 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 forma- 

 tion, 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 dura- 



