of Geological Periods, 349 



Liriodendron Meckii, Heer, indicated by Dr. Heer in the Upper 

 Chalk of Nebraska, associated with Magnolice^ and other i)i- 

 cotyledons, among which the learned professor of Zurich thought 

 he could recognize the genera Populus, Salia;, and PlatanuSy 

 although too doubtfully to allow their presence to be positively 

 affirmed. 



We must therefore stop at this latter limit and close this long 

 examination. Leaving all theory out of the question, it appears, 

 from the combined progress of the tropical and European types, 

 that these two categories have coexisted for a long time without 

 eliminating each other, but simply in juxtaposition. The time 

 during which this juxtaposition lasted was much longer than 

 has hitherto been supposed. It extends from the extreme base 

 of the Tertiary series nearly to the close of the Swiss Mollasse. 

 In fact it is only at this epoch that the tropical types decline 

 and are gradually eliminated by the genera which have remained 

 proper to the boreal zone, and of which the preponderance in- 

 creases in the same proportion. Between these limits the two 

 groups live associated together without any great variation in the 

 physiognomy and relative proportions of the indigenous group, 

 although its part must sometimes have diminished to such an 

 extent as to deprive us of the traces of its existence, or at least 

 to render them very rare. 



III. 



All that is necessary now is to sum up the preceding observa- 

 tions, so as to draw general conclusions from them. 



If we consider plants alone, geological time may be divided 

 pretty naturally into a certain number of great phytological 

 periods. 



In the first and most distant we cannot indicate with certainty 

 any of the existing genera : Dicotyledons and Monocotyledons 

 are absent; we observe exclusively Vascular Cryptogamia and 

 Gymnospermia ; a portion only of these plants enter into still 

 existing families ; the indications derived from the observation 

 of these seem to announce the existence of a warm, humid, 

 equable climate, subjected to uniform conditions all over the 

 globe. 



In the second period, which includes the Triassic, the Jurassic, 

 and a part of the Cretaceous epochs, the character of the vege- 

 tation changes sensibly. We can already indicate a small num- 

 ber of genera identical with those of the existing world; the 



* The persistence of the characteristic types of the present temperate 

 regions in the secondary formations is also attested by cones of the 

 genus Cedrus, in admirable preservation, observed by M. Heer in the 

 chalk of Moleteiu in Moravia. 



