Palaeontology. 201 



Planorbis. The following is an authoritative rtsumt 

 of the facts. 



As the deposits seem to have been continuous for ages, and 

 the fossil shells very abundant, this seemed to be an excellent 

 opportunity to test the theory of derivation. With this end in 

 view, they have been madf the subject of exhaustive study by 

 Hilgendorf in 1866, and by Hyatt in 1880. In passing from the 

 lowest to the highest strata the species change greatly and many 

 times, the extreme forms being so different that, were it not for 

 the intermediate forms, they would be called not only different 

 species, but different genera. And yet the gradations are so in- 

 sensible that the whole series is nothing less than a demon- 

 stration, in this case at least, of origin of species by derivation 

 with modifications. The accompanying plate of successive forms 

 (Fig. 89), which we take from Prof. Hyatt's admirable memoir, 

 will show this better than any mere verbal explanation. It will 

 be observed that, commencing with four slight varieties pro- 

 bably sexually isolated varieties of one species, each series 

 shows a gradual transformation as we go upward in the strata 

 i. e. onward in time. Series I branches into .three sub-series, in 

 two of which the change of form is extreme. Series IV is re- 

 markable for great increase in size as well as change in form. 

 In the plate we give only selected stages, but in the fuller plates 

 of the memoir, and still more in the shells themselves, the sub- 

 tilest gradations are found *. 



Here is another and more recently observed case of 

 transmutation in the case of mollusks. 



The recent species, S trombus accipitrinus y still in- 

 habits the coasts of Florida. Its extinct prototype, 

 5. Leidy, was discovered a few years ago by Prof. 

 Heilprin in the Pliocene formations of the interior 

 of Florida. The peculiar shape of the wing, and 

 tuberculation of the whorl, are thus proved to have 

 grown out of a previously more conical form of 

 shell. 



1 Le Conte, loc. tit., pp. 236-7. 



