Allard 



— US- 



Length of Day in the Past 



If, as the writer's theory postulates, a 12-hour day with warm tempera- 

 tures finally became established over all the earth, all these plants could 

 thrive as far poleward as temperature conditions would allow, and it is 

 possible that therefore they may have thrived at the poles at the close of 

 the Cretaceous Period. However, any change toward cooler conditions 

 and longer days in high latitudes would have caused a quick recession of all 

 these shrubs into the tropics. While the Poinsettia and the Bougainvillea 

 would have been driven southward by the combined operation of two power- 

 ful climatic factors, namely, low temperatures and unfavorably long days, 

 the Turkscap Hibiscus would have been brought to extinction only by the 

 factor of cold, owing to its day-neutral constitution. Unquestionably there 

 are thousands of other woody shrubs and even forest trees with similar 

 narrow temperature and length-of-day characteristics, which rigidly con- 

 fine them to tropical conditions of warmth and short days. These instances 

 demonstrate how readily many species of a tropical flora growing over all 

 the earth would suffer quick extinction were the climatic conditions of 

 warmth and constant short days replaced by cold temperatures and long 

 summer days in any geological period. 



A number of shrubs, at present found in higher latitudes, appear to be 

 adapted only to lengths of day in excess of the 12-hour photoperiods prev- 

 alent in equatorial regions. These are Althaea, Hibiscus syriacus, and 

 Spice Bush, Benzoin aestivale. Tests have shown that neither of these 

 shrubs was able to flower in regions where the length of day was perma- 

 nently reduced to 12 hours. Such rigid limitations in length of day would 

 confine these shrubs to regions and climatic zones where the summer length 

 of day would exceed 12 hours, so far as sexual reproduction is concerned. 

 Such forms of these species as have been studied would find no place in 

 a climate characterized by a uniform 12-hour day. 



In harmony with the present views of most geologists it is assumed that 

 during the Cretaceous time, some combination of astronomic relations sus- 

 tained a climate of marked uniformity characterized by warm temperatures 

 and slight zonal differentiations over much of the earth. Such conditions 

 of climate can hardly be postulated without assuming also an approach to a 

 world-wide uniformity in length of day associated perhaps with a condi- 

 tion where the obliquity of the axis has approached zero over a great period 

 of time. This position of the terrestrial axis with reference to the plane of 

 its orbit would sustain a length of day from sunrise to sunset of about 12 

 hours over all the earth, with the exception perhaps of an extremely small 

 area immediately around the Poles. ^ 



Conclusions : — It is conceded by all students of ancient life that 

 climate has always been a powerful factor in modifying the primitive vege- 

 tation. Temperature has been given the major role in the climatic complex 

 in all theories purporting to explain secular changes in the primitive floras 



^ At the present time with the earth's axis inclined about 23° 27' 3" and in the peri- 

 helion portion of its orbit, the days and nights become nearly equal over all the earth, 

 only at the time of the equinoxes about March 21 and September 21. Assuming a 



vertical axis, and the earth being an oblate spheroid, with flattening at the Poles, the 

 rays of the sun always having a position vertical to the equator at noon would suffer 



their extreme refraction and shine upon the Poles although the sun might be actually 

 below the horizon here. 



