418 THE ORIGIN OF SPECIES 



measured by years, have probably been short in comparison with 

 the periods during which 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 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 duration has probably been shorter 

 than the average duration of specific forms. Successive forma- 

 tions are in most cases separated from each other by blank in- 

 tervals of time of great length, for fossiliferous formations thick 

 enough to resist future degradation can, as a general rule, be ac- 

 cumulated 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 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 beneath 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 rela- 

 tive positions, we have no reason to assume that this has always 

 been the case; consequently formations much older than any now 

 known may lie buried beneath the great oceans. With respect to the 

 lapse of time not having been sufficient since our planet was con- 

 solidated for the assumed amount of organic change, and this 

 objection, as urged by Sir William Thompson, is probably one of 

 the gravest as yet advanced, I can only say, firstly, that we do 

 not know at what rate species change, as measured by years, and 

 secondly, that many philosophers are not as yet willing to admit 

 that we know enough of the constitution of the universe and of 

 the interior of our globe to speculate with safety on its past dura- 

 tion. 



That the geological record is imperfect, all will admit; but 

 that it is imp '^rf ect to the degree required by our theory, few will 

 be inclined to admit. If we look to long enough intervals of time, 

 geology plainly declares that species have all changed; and they 

 have changed in the manner required by the theory, for they have 

 changed slowly and in a graduated manner. We clearly see this in 

 the fossil remains from consecutive formations invariably being 



