S36 
GEOLOGICAL SUCCESSION. 
Chap. X. 
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. The best definition probably is, 
that the higher forms have their organs more distinctly 
specialised for different functions ; and as such division of 
physiological labour seems to be an advantage to each 
being, natural selection will constantly tend in so far to 
make the later and more modified forms higher than their 
early progenitors, or than the slightly modified descend¬ 
ants of such progenitors. In a more general sense the 
