298 
iMPEEFECTIO^iT OE THE 
Chap. IX. 
of B and C, and yet might not at all necessarily be 
strictly intermediate between them in all points of struc¬ 
ture. So that we might obtain the parent-species and 
its several modified descendants from the lower and 
upper beds of a formation, and unless we obtained 
numerous transitional gradations, we should not recog¬ 
nise their relationship, and should consequently be com¬ 
pelled to rank them all as distinct species. 
It is notorious on what excessively slight differences 
many palaeontologists have founded their species; and 
they do this the more readily if the specimens come 
from different sub-stages of the same formation. Some 
experienced conchologists are now sinking many of the 
very fine species of D’Orbigny and others into the rank 
of varieties; and on this view we do find the kind of 
evidence of change which on my theory we ought to 
find. Moreover, if we look to rather wider intervals, 
namely, to distinct but consecutive stages of the same 
great formation, we find that the embedded fossils, 
though almost universally ranked as specifically dif¬ 
ferent, yet are far more closely allied to each other than 
are the species found in more widely separated forma¬ 
tions ; but to this subject I shall have to return in the 
following chapter. 
One other consideration is worth notice: with animals 
and plants that can propagate rapidly and are not 
highly locomotive, there is reason to suspect, as we 
have formerly seen, that their varieties are generally at 
first local; and that such local varieties do not spread 
widely and supplant their parent-forms until they have 
been modified and perfected in some considerable de¬ 
gree. According to this view, the chance of discovering 
in a formation in any one country all the early stages 
of transition between any two forms, is small, for the 
successive changes are supposed to have been local or 
