DISTRIBUTION IN TIME AND SPACE. 55 



lusks, where the differences are much greater than those which 

 palaeontologists, seeking distinctive characters for their periods 

 or formations, have been accustomed to consider as of specific 

 and even generic value. It is not many years since some of the 

 best paleontologists held, and actively worked upon, the opinion 

 that each principal geological period closed with a cataclysm 

 involving the destruction of all life, and a subsequent re-creation ; 

 and many hundreds of species derive their only title to validity 

 from this hypothesis. It is no reproach to able and honest palae- 

 ontologists that they have in this manner gone estray ; nor have 

 they erred more unfortunately than some conchologists, who 

 have not hesitated to describe as new identical forms of recent 

 mollusca, simply because they occupy different geographical 

 areas. How many identical species have been described under 

 different names when occurring on the opposite coasts of Central 

 America, under the belief that, however similar they could not 

 have a common origin ? an error thoroughly dispelled by the 

 researches of Gabb and others upon the geolog}^ and palaeon- 

 tology of Central America and the Caribbean area. 



Bronn has also prepared a table of the number of species of 

 each genus of prosobranchiates appearing in the various forma- 

 tions, with the totals of species, fossil and recent appertaining to 

 each. As in his table just quoted, much allowance must be made 

 for actual and relative increase of species made known since his 

 publication. The Genera are within the Lamarckian limits, and 

 those with which we are at present occupied will.be found near 

 the bottom of the table. 



