384 RECAPITULATION. [CHAP. X^ 



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 under- 

 gone modification, though long as measured by years, have pro- 

 bably 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 dis- 

 covery 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 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 f onnations 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 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 

 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 duration. 



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 

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



