SUMMARY. 119 



expansion of the Arietidse in the Bucklandi zone of Central Europe, the rapidity 

 with which the forms of the still later beds must have come into being in order 

 to be presented in a body, as in the Tuberculatus beds of the C5te d'Or, and the 

 limited thickness of the beds, are all against the supposition that it required vast 

 periods of time for a species to become modified and give rise to series of distinct 

 forms. Either the species of the Arietidas had time enough during the deposi- 

 tion of the Planorbis, Angulatus, and Bucklandi beds of the Lower Lias to spread 

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

 all the series described above, or else the same species and genera had invariably 

 distinct centres of origin in the different basins. One might support the latter 

 view and favor polygenesis even in this extreme sense with considerable show 

 of reason, if there were not such a mass of evidence in favor of migration, some of 

 which we have given above. If there were space, we 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 

 more striking examples are, however, quite well known, and hardly need to be 

 dwelt upon. 



