Chap. XV. Recapitulation. 409 



without leaving modified descendants; and the periods, during 

 which species have undergone modification, though long as mea- 

 sured hy years, have probably been short in comparison with the 

 periods during which they retained the same form. It is the domi- 

 nant 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 formations 

 are in most cases separated from each other by blank intervals 

 of time of great length; for fossiliferous formations 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 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 relative 

 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 consolidated 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 imperfect 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 

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



