J. LeConte—Critical Periods in the History of the Earth. 111 
Similarly molluscous shells migrated slowly southward and 
again northward to their present position. But plants and 
some terrestrial invertebrates, such as znsects, had an alternative 
similar migrations among mammals also. ut it is evident 
that while plants and invertebrates might endure such changes 
element of very rapid local change, viz: the znvaszon of one fauna 
by another equally well adapted to the environment, and the 
Struggle for life between the invaders and the autochthones. 
the two Americas were certainly separated by sea in the region 
in : as . application, with reference to Mount Washington and other Arctic insects 
th e ca, was previously made by Prof. A. 8. Packard, Jr., in the Memoirs of 
© Soston Soc. Nat. Hist., i, p. 256, 1867.—Eps. 
