Questions as to the Origin and Kectinction of Species. 418 
Atlantic. The similarity of the Carboniferous flora on the two 
sides of the Atlantic, and thé great number of identical species 
proves a still closer connection in that period. These coinci. 
dences are too extensive and too frequently repeated to be the 
result of any accident of similar sequence at different times. 
differences in the features of each period, as, for instance, the 
floras of the Lower and Upper Devonian, and of the Lower, 
Middle, and Upper Cdebivndideeaits 
nother geographical question is that which relates to cen- 
Further, it is possible that these changes of subsidence ma 
ve some connection with the introduction, as well as wi 
the extinction, even of specific types. It is certain, at least, in 
the case of land plants, that such types come in most abun- 
tly immediately after elevation, though they are most abun- 
dantly preserved in periods of slow subsidence. I do not mean, 
however, that this connection is one of cause and effect; there 
are, indeed, indications that it is not so. One of these is, that 
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ain simi 
| _ lar to those of the Carboniferous. Of these types a few only 
_ Te-appear in the Middle Coal formation under identical forms ; 
_ 4 great number appear under allied forms; some altogether dis- 
- 4ppear. The Erian flora of New Brunswick and Maine occurs 
ide by side with the Carboniferous of the same region ; so does 
_ the Erian of New York and Pennsylvania with the Carbonif- 
us of those States. Thus we have data for the comparison of 
n direct seq f 
the Lower, Middle, and Upper Erian, and the Lower, 
