- MMARY. 119 



Lrietidae in the Bucklandi tone of Central Europe, the rapidity 

 with which the forms of tli«' >t ill later beds mual have come into being in order 



presented in a body, a* in the Tuberculatum l>i<l> «>i th< I ( >r, and the 



limited thickness of the beds, are all against the supposition thai it required vas( 

 periods i>t time for a Bpecies i" become modified and -/w- ris( of distinct 



forms. Either the Bpecies <>t the Arietidoa had time enough during the •■ 

 tion of the Piano . ilatus, and Bucklandi beds of tin' Lower Lias to Bpread 



themselves over the entire area of modern Europe, and generate from one form 

 all the scribed above, or else the same Bpecies and genera had invariably 



distin. i rigin in the different basins. One might Bupporl the latter 



ind favor polygenesis even in this extreme sense with considerable bIiow 

 of reason, if there were not Buch a mass <>f evidence in favor of migration, some <>f 

 which «<• have given above. If there were space, \\<- could add examples from 

 the researches of various well known zoologists upon the migrations and modi- 

 fication of species in modern times, both along the coasts and over the land. The 

 samples are, however, quite well known, and hardly need t<> l>e 

 dwelt upon. 



