452 RECAPITULATION. 



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 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 un- 

 dergo 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 peri- 

 ods during which they retained the same form. It is the 

 dominant and widely ranging species which vary most fre- 

 quently 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 consid- 

 erably 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 duration has probably been shorter 

 than the average duration of specific forms. Successive 

 formations are in most cases separated from each other by 

 blank intervals of time of great length, for fossiliferous for- 

 mations thick enough to resist future degradation can, as a 

 general rule, be accumulated only where much sediment is 

 deposited on the subsiding bed of the sea. During the alter- 

 nate periods of elevation and of stationary level, the record 

 will generally be blank. During these latter periods there 

 will probably be more variability in the forms of life; during 

 periods of subsidence, more extinction. 



With respect to the absence of strata rich in fossils be- 

 neath the Cambrian formation, I can recur only to the 

 hypothesis given in the tenth chapter; namely, that though 

 our continents and oceans have endured for an enormous 

 period in nearly their present relative positions, we have no 

 reason to assume that this has always been the case ; conse- 

 quently formations much older than any now known may lie 



