Allard —109— Length of Day in the Past 



ern latitudes. All these herbaceous plants would become an important 

 group where formerly they were suppressed or occupied a minor place in 

 competition with the larger and more vigorous trees and shrubs, as these 

 less aggressive plants do today. It is obvious, then, that these cooler north- 

 ern regions, more especially, because of the advances and retreats of the 

 vast ice sheets here, became favorable land areas for the development of a 

 great herbaceous Angiospermous flora. This appears to have been the final 

 status of plant life in Tertiary time. 



It is true that one must assume certain niceties of astronomic conditions, 

 involving perhaps a vertical terrestrial axis, with a subsequent generation 

 of wobble and obliquity of this axis, to develop season changes and even the 

 glaciation stages themselves. This, however, seems no more unreasonable 

 than many of the fanciful theories already promulgated to explain the 

 great Glaciations of the Pleistocene Period, not one of which has been gen- 

 erally accepted as final by all geologists. 



This hypothesis does not attempt to prove that herbs actually evolved 

 from the great woody Angiospermous flora of the Cretaceous period by 

 some direct response involving suppression or reduction of the continuous 

 cambium elements of the stem with the production of intrafascicular par- 

 enchyma. It simply assumes that the herbs were already in existence at 

 that time, but owing to the dominance of the woody forms, were in no posi- 

 tion to compete with these, and, therefore, perhaps existed as a ground her- 

 bage for the most part in the great Angiospermous forests. Only the day- 

 neutral and the short-day forms could have found the daily conditions of 

 light duration favorable to their existence in Cretaceous time even under 

 the most congenial habitat conditions if a 24-hour cycle prevailed with 

 nearly equal durations of day and night. 



This hypothesis appears to account for the great multiplication of 

 Angiospermous herbs of the Tertiary Period, and their rise to subsequent 

 numerical superiority in the temperate and colder regions of the earth as 

 we find them, also, today. 



It must be remembered that every geological cycle since the Cambrian 

 Period has involved many millions of years, and incomprehensible periods 

 of time have been required for each geological time period. While the 

 geologist professes to speak in terms of years for the Triassic Period, the 

 Jurassic, the Cretaceous or other periods, which have preserved their fossils 

 for his studies, the gaps between these also probably represent many more 

 millions of years of lost records. To measure conditions, then, of the 

 earth's wobble and the inclinations of the axis at the present moment, and 

 to hold that this has been always the actual condition even billions of years 

 ago appears to be a rather dogmatic view which cannot but be regarded with 

 suspicion. 



It has been assumed that the herbs were already existent at the close of 

 the Cretaceous Period, and there is reason to believe that herbaceous types 

 of plant life are quite as ancient as the woody types. If it is true that the 

 herbaceous type of plant is better adapted to cold climates, as its behavior 

 indicates today, both in annual and in perennial forms, this life form must 

 always have found favorable conditions for its existence even in past geo- 

 logical eras. Lofty mountains very early appeared on the great land areas 



