194 H. B. GCPPY, M.B., IMf.S.b;., ON PLANT-DISTPJEUTION 



that is especially discussed by Cliamberlin and Salisbury in 

 their recent great work on Geokigy. Whatever may be the 

 opinions of these two American authors concerning the 

 occurrence of a universal warm climate during the early 

 geological periods, a notion that they reject altogether, they 

 hold no uncertain views relating to the differentiation of 

 climates and horas in later ages. The same mixed flora, in 

 which plants now confined to separate tropical and temperate 

 regions were associated, extended, as they remark, in Upper 

 Cretaceous times in Europe and North America over thirty-five 

 degrees of latitude, reaching as far north as Greenland. Such 

 a flora, they suggest, would imply "climates of a less differ- 

 entiated or less diversified nature." These mixed or undiffer- 

 entiated floras also occurred in the Eocene, and the authors la}' 

 stress more than once on the association in the deposits of this 

 age of palms and poplars. "Probably the true view," they 

 write, "is that the mixed or undifferentiated flora of the 

 Cretaceous and Eocene, when it came to be subjected later to 

 severe climatic and other crucial conditions, became modified 

 into adaptive grou])s, some of which came to be restricted to 

 the tropical regions and are now known as tropical plants, 

 others to the temperate, and still others to the boreal regions, 

 acquiring corresponding designations." In another place they 

 term this process " adaptive differentiation " (vol. iii, pp. 226-7). 



The process of the dissociation of the mixed floras extended, 

 as they observe, into the Miocene, when occurred " the gradual 

 removal to the south of the forms now regarded as tropical or 

 subtropical, and the concentration at the north of the forms 

 that now characterise those latitudes." Here they are undecided 

 as to whether this was the result of " natural differentiation 

 and segregation of the previously mixed forms " or of " a pro- 

 gressive differentiation of climate " (vol. iii, p. 283). However, 

 they leave us no longer in doubt in the matter when writing of 

 the continued dissociation of the mixed floras in the succeeding 

 Pliocene period. "The Pliocene (they observe) was character- 

 ised by a still further sorting out of the mixed flora of previous 

 periods and by the southerly migration of wliat are now tropicul 

 and sub-tropical ])lants." The evidence, as they proceed to 

 show, indicates not only " a general differentiation " of plants but 

 also that " the climate was becoming differentiated, and on the 

 whole cooler than it had been in earlier Tertiary periods " (vol. 

 iii, pp. 320-1). 



And now, in conclusion, I think I may claim to have shown 

 that, as far as the cited opinions indicate, the differentiation 



