Chap. IX. 
GEOLOGICAL KECOKD. 
293 
adjoining shoal parts of the sea will be increased, and 
new stations will often be formed;—all circumstances 
most favourable, as previously explained, for the form¬ 
ation of new varieties and species; but during such 
periods there will generally be a blank in the geological 
record. On the other hand, during subsidence, the 
inhabited area and number of inhabitants will decrease 
(excepting the productions on the shores of a continent 
when first broken up into an archipelago), and conse¬ 
quently during subsidence, though there will be much 
extinction, fewer new varieties or species will be formed; 
and it is during these very periods of subsidence, that 
our great deposits rich in fossils have been accumulated. 
Nature ^may almost be said to have guarded against 
the frequent discovery of her transitional or linking 
forms. 
From the foregoing considerations it cannot be doubted 
that the geological record, viewed as a whole, is ex¬ 
tremely imperfect; but if we confine our attention to 
any one formation, it becomes more difficult to under¬ 
stand, why we do not therein find closely graduated 
varieties between the allied species which lived at its 
commencement and at its close. Some cases are on 
record of the same species presenting distinct varieties 
in the upper and lower parts of the same formation, but, 
as they are rare, they may be here passed over. Al¬ 
though each formation has indisputably required a 
vast number of years for its deposition, I can see several 
reasons why each should not include a graduated series 
of links between the species which then lived; but I can 
by no means pretend to assign due proportional weight 
to the following considerations. 
Although each formation may mark a very long lapse 
of years, each perhaps is short compared with the period 
requisite to change one species into another. I am 
