| res 
SUMMARY. 119 
expansion of the Arietidee 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 Cote 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. Hither 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 zodlogists 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. 
