336 GEOLOGICAL SUCCESSION. Chap. X. 



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 in- 

 habiting the sea, have changed almost simultaneously 

 throughout the world, and therefore under the most 

 different climates and conditions. Consider the pro- 

 digious vicissitudes of climate during the pleistocene 

 period, which includes the whole glacial period, 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 

 fact of fossil remains from closely consecutive forma- 

 tions, though ranked as distinct species, being closely 

 related, is obvious. As the accumulation of each for- 

 mation 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 ap- 

 peared 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 spe- 

 cies ; and these we assuredly do find. We 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 

 has been much discussion whether recent forms are 

 more highly developed than ancient. I will not here 

 enter on this subject, for naturalists have not as yet 

 defined to each other's satisfaction what is meant by 

 high and low forms. But in one particular sense the 



