Allard — 117 — Length of Day in the Past 



It is assumed that the Tertiary change in climate interrupted the Cre- 

 taceous uniformities of warm temperature, weak zonal distinctions, and a 

 world-wide uniformity of length of day, which had persisted at or near 12 

 hours in duration. With the establishment of long days in the Tertiary 

 cycle, great numbers of woody plants flowering only in response to such 

 short days became extinct, or were driven southward to find a home only 

 in tropical regions where short days would still persist. On the other hand 

 a great group of long-day plants that had been completely suppressed over 

 all the earth during the Cretaceous Period by the world-wide prevalence 

 of the short 12-hour day would now evolve in northern latitudes. While 

 the long-day herbs would be confined to the higher latitudes the short-day 

 Angiospermous herbs would readily adapt themselves to the seasonal sum- 

 mer cycle and flourish well toward the Poles becoming fall-flowering plants 

 as at present. The day-neutral plants would thrive anywhere over the earth 

 where habitat conditions favored their existence, whether the days were 

 only 12 hours long as at the equator or where conditions of continuous 

 light in summer would prevail, as at the Poles. Only day-neutral plants 

 and long-day plants capable of flowering in response to continuous light 

 would find Polar summers favorable to their existence. 



Similar conditions of climate occurring also in the southern hemisphere 

 involving seasonal changes causing colder temperatures and longer summer 

 days intervening between the equinoxes would likewise induce the rise of 

 an Angiospermous herbaceous element here. Owing to less rigorous con- 

 ditions of cold, however, the original proportions of the woody Cretaceous 

 flora would suffer less change, and a smaller assemblage of herbaceous 

 Angiosperms would be called into expression. 



It is known that in many instances the same genera and even the same 

 species of plants occur in both hemispheres, and this has been explained by 

 SiNNOTT and Bailey and others as due mainly to the dispersal of forms 

 from the great northern reservoirs of herbaceous Angiosperms which arose 

 to prominence in Tertiary time. These similarities in many instances may 

 perhaps be in part explained by the assumption that they represent parallel 

 lines of derivation descending from an ancient Cretaceous flora character- 

 ized by great similarity of forms the world over. Since the primitive as- 

 semblages were similar in Cretaceous time, and the conditions of climatic 

 change in Tertiary time were of the same order in the higher latitudes, with 

 respect to temperature and length of day, a certain degree of similarity 

 should be expected in the descendants of the plant life of the two hemis- 

 pheres. 



The geographic dispersal of the Tertiary herbaceous Angiosperms from 

 the higher latitudes where they first attained their dominance must have been 

 influenced not only by the temperature factor but also by the length-of-day 

 factor of cHmate. 



The great equatorial regions with their permanence of high tempera- 

 tures, their dominant climax forests of woody Angiosperms where high 

 rainfall prevailed, and the permanently established short days, were power- 

 ful barriers to the dispersal into the southern hemisphere of many herba- 

 ceous Angiosperms. Even though there was a continuity of high moun- 

 tain chains in western North and South America, which appeared to fur- 



