Allard — 107— Length of Day in the Past 



tion of many species of plants that were not particularly sensitive to narrow 

 ranges of temperature. 



Changes in Plant Life in Tertiary Time: — Inferences regarding the 

 climate of a given era or period must be drawn with great caution, yet lead- 

 ing authorities have come to more or less agreement on many points. It 

 is considered by Berry and others that certain features of the Cretaceous 

 climate were quite unlike those of today, namely that the earth was much 

 warmer, with much less seasonal change and zonal differentiation. These 

 conclusions are drawn from various sources including the indistinct growth 

 rings in the trees of this time, and the presence of a tropical and subtropical 

 vegetation even in Greenland and in northern Alaska. In Eocene time 

 which marks the earliest period of the Tertiary, conditions were very differ- 

 ent from those which we experience today, although there is reason to be- 

 lieve that climatic and floral changes intensified in later periods of the 

 Tertiary were being foreshadowed here. 



Whereas in the Cretaceous Period, as previously stated, the Angiosper- 

 mous forms showed an intermingling of what today appear to be tropical 

 and temperate region species, even in the highest latitudes as revealed by 

 fossil records, a progressive dissociation of these finally became evident, 

 indicating a climate evolving into more strongly zonal distributions of 

 temperature accompanied by changes in seasonal length of day. It seems 

 remarkable that warm temperature conditions of this character could havc 

 prevailed in these high latitudes to sustain this type of flora here even in 

 the Miocene, especially when only a little later in geological time great ice 

 sheets had developed and covered these once warm regions. The tropical 

 or subtropical types that had flourished in high latitudes gradually disap- 

 peared here, and an increasing number of deciduous forms made their ap- 

 pearance. 



Pliocene plant life, as indicated by the fossil remains gave evidence of 

 still more rigid sorting of the flora into tropical and temperate elements, 

 which had begun in previous epochs so that tropical and subtropical plants 

 were pushed even further southward toward their present equatorial limits. 

 This separation of plant types appears to have proceeded more rapidly in 

 North America than in Europe. These southerly colonizations of par- 

 ticular elements of the plant life that were formerly strangely commingled 

 in far northern and now frigid regions have been interpreted as pointing 

 to a gradual refrigeration of climate extending into lower latitudes, with 

 consequent modification of the plants into new and specific adaptations by 

 the changing climate. 



It is not necessary to conclude that there was a progressive cooling of 

 the entire earth's surface due to secular loss of heat from its mass. The 

 evidence, on the contrary, points rather to some temporary climatic oscilla- 

 tion which may have resulted from events connected with the relations of 

 the earth's axis to the plane of the ecliptic, to changes in the ellipticity of 

 the earth's orbit, or to other unknown conditions. Whatever the cause it 

 appears obvious that the world climate, formerly warm and uniform or ap- 

 proaching this condition in Cretaceous time, had become gradually differ- 

 entiated into zonal distributions of heat and coolness, and this evolution 

 continuing until the Pleistocene epoch, spread great ice sheets over the 



