Chap. XI.] AFFINITIES OF EXTINCT SPECIES. 115 



will not attempt to account for the close resemblance of 

 distinct species in closely consecutive formations, by 

 the physical conditions of the ancient areas having 

 remained nearly the same. Let it be remembered that 

 the forms of life, at least those inhabiting the sea, have 

 changed almost simultaneously throughout the world, 

 and therefore under the most different climates and con- 

 ditions. Consider the prodigious vicissitudes of climate 

 during the pleistocene period, which includes the whole 

 glacial epoch, and note how little the specific forms of 

 the inhabitants of the sea have been affected. 



On the theory of descent, the full meaning of the 

 fossil remains from closely consecutive formations being 

 closely related, though ranked as distinct species, is 

 obvious. As the accumulation of each formation has 

 often been interrupted, and as long blank intervals have 

 intervened between successive formations, we ought not 

 to expect to find, as I attempted to show in the last 

 chapter, in any one or in any two formations, all the 

 intermediate varieties between the species which 

 appeared at the commencement and close of these 

 periods : but we ought to find after intervals, very long 

 as measured by years, but only moderately long as 

 measured geologically, closely allied forms, or, as they 

 have been called by some authors, representative species; 

 and these assuredly we do find. We find, in short, such 

 evidence of the slow and scarcely sensible mutations of 

 specific forms, as we have the right to expect 



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