GEOLOGICAL SUCCESSIOxX 301 



each other, than are the fossils from two remote forma- 

 tions. Pictet gives as a well-known instance, the 

 general resemblance of the organic remains from the 

 several stages of the Chalk formation, though the 

 species are distinct in each stage. This fact alone, 

 from its generality, seems to have shaken Professor 

 Pictet in his firm belief in the immutability of species. 

 He who is acquainted with the distribution of existing 

 species over the globe, will not attempt to account for 

 the close resemblance of the 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 simul- 

 taneously throughout the world, and therefore under 

 the most different climates and conditions. Consider 

 the prodigious vicissitudes of climate during the pleisto- 

 cene period, which includes the whole glacial period, 

 aud 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 

 fact of fossil remains from closely consecutive forma- 

 tions, though ranked as distinct species, being closely 

 related, 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 

 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 we assuredly 

 do find. \Ve find, in short, such evidence of the slow 

 and scarcely sensible mutation of specific forms, as we 

 have a just right to expect to find. 



On the state of Development of Ancient Forms. — There 



