418 ON THE ORIGIN OF SPECIES 



discover, owing to the imperfection of the geological 

 record. Numerous existing doubtful forms could be 

 named which are probably varieties ; but who will pre- 

 tend that in future ages so many fossil links will be 

 discovered, that naturalists will be able to decide, on 

 the common view, whether or not these doubtful forms 

 are varieties ? As long as most of the links between 

 any two species are unknown, if any one link or inter- 

 mediate variety be discovered, it will simply be classed 

 as another and distinct species. 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. Widely 

 ranging species vary most, and varieties are often at 

 first local, — both causes rendering the discovery of 

 intermediate links less likely. Local varieties will not 

 spread into other and distant regions until they are con- 

 siderably modified and improved ; and when they do 

 spread, if discovered in a geological formation, they 

 will 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, I am inclined to believe, has been shorter 

 than the average duration of specific forms. Successive 

 formations are separated from each other by enormous 

 blank intervals of time ; for fossiliferous formations, 

 thick enough to resist future degradation, can be 

 accumulated only where much sediment is deposited on 

 the subsiding bed of the sea. During the alternate 

 periods of elevation and of stationary level the record 

 will 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 fossiliferous forma- 

 tions beneath the lowest Silurian strata, I can only 

 recur to the hypothesis given in the ninth chapter. 

 That the geological record is imperfect all will admit ; 

 but that it is imperfect to the degree which I require, 

 few will be inclined to admit. If we look to long 

 enough intervals of time, geology plainly declares that 



