184 INTRODUCTION. 
and which was described by the late Jacob Green, M. D., 
as Helicina rubella. We cannot doubt the identity of 
the two, as they are as nearly alike as the soft and 
crumbling fossil can be to the shell whose surface yet 
retains its original character. The recent shell is ex- 
tremely rare, and is found in only a few collections, and 
in this respect offers a strong contrast to the fossil, which 
must have been very numerous. One of the most evi- 
dent facts taught to us by geology is the constant suc- 
cession of zoological species. They come into existence 
one after another, increase and flourish for a longer or a 
shorter time, and then gradually die out and disappear. 
That there are laws which regulate and limit their con- 
tinuance we cannot doubt, although they are not under- 
stood by us ; neither can we doubt that they are yet in 
operation, and that in accordance with them species do 
now sometimes become extinct. The abundance of the 
species under consideration, at the time of its deposition 
in the fossil beds, and its rarity now, suggest the 
thought, that having then reached its greatest numerical 
development, it has since slowly declined, and is now in 
a condition of zoological senility antecedent to its entire 
extinction. 
The light which is afforded by geology, in elucidating 
the former zoological condition of the earth, is a beauti- 
ful illustration of the manner in which one science often 
ministers to another. Without the aid which has been 
derived from our knowledge of the animated creation, 
geology, indeed, would hardly have attained the cer- 
