32 College of Forestry 
vegetation of today, if not in great numbers and diversity of 
forms, still as prominent forest forming species. And their 
segregation generally as boreal conifer forests leads to the 
suggestion that this segregation has been in effect from an 
early period in the history of modern conifers and makes it 
pertinent to call attention to the suggestion of Gothan’* that 
in the later Jurassic there began a gradual differentiation of 
climate which toward the poles was manifested in alternating 
seasons. ‘This condition, indicated among other evidences by | 
the presence of true annual rings of growth in the gymno- 
sperms of higher latitudes, resulted in a more or less obvious 
segregating of floral provinces, the more and more dominant 
gymnosperms of the group Abietineae (the present most nu- 
merously represented group of gymnosperms) extending into 
higher latitudes. At this time the cyeads (as always, ap- 
parently, tropical species) reached only perhaps to 70 de-: 
grees north latitude, the approximate limit of tropical 
climate. 
While we may at least say that at this landmark we find 
slight beginnings of modern types and possibly of modern 
climatic differentiation, still it is a distinct and remote vege- 
tation age, made the more notably so in that no trace or fore- 
east of our now dominant angiosperm vegetation seems to 
have existed. 
Plant Life of the Later Cretaceous Period. Introduction 
of Modern Types of Broad-leaved Dicotyledonous Trees. 
Let us turn next to the later or upper Cretaceous period 
toward the end of the Mesozoic era. Again radical and in 
1Gothan, Die Frage der Klimadifferenzierung im Jura und in der 
Kreideformation. Jahrb. K. preuss. Geo. Landesanst. 19:1908, 2-220. 
In this connection it is desirable to call attention to the theories of 
Chamberlin as to alternating periods of climate in geologic time. (Jour. 
Geo. vols. v—viii:1897—1901.) and to the discussion of animal evolution 
as determined or influenced thereby, by W. D. Matthew. (Climate and 
Evolution. Ann. N. Y. Acad. Sci. 24:1915, pp. 171-318). Chamberlin’s 
theories “involve an alternation of climates through the course of 
geologic time from extremes of warm, moist tropical and uniform, to 
extremes of cold, arid, zonal climates” (Matthew, loc. cit., p. 173). 
