184 INTRODUCTION. 



and which was described by the late Jacob Green, M. D., 

 as HeUcina 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 tliis respect oifers 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 mider 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 entii'e 

 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- 



