356 ¥. H. KNOWLTON C'KITEKIA 1<X)R DETERMINATION OF CLIMATE 



where decay is more rapid, not only must the humidity be greatly increased 

 in order to provide the necessary wetness to retard decomposition, but there 

 must be no long dry season of the year for the too great reduction of the water 

 cover. The observations of peat formation at the present day in tropical cli- 

 mates show that in order to permit the deposition of peat the rainfall must be 

 both very heavy and fairly well distributed through the entire year. 



"3. Great radial distribution, seemingly over the greater part of the earth, 

 and especially over wide ranges of latitude, of identical species and genera in 

 characteristic association, indicating the extension of approximately uniform 

 climatic conditions in these regions. Floras identical, or essentially identical, 

 in remote or detached regions can owe their identity to no other cause than 

 approximately continuity of the environment, whether that continuity is geo- 

 graphic or chronographic. Conversely, migration of a flora without change is 

 possible only through regions of essentially identical environmental conditions. 

 Illustrations are found in the Carboniferous, Triassic, Jurassic, and Lower 

 Cretaceous floras, and even to a remarkable degree in the Upper Cretaceous 

 and Tertiary floras. 



"4. Presence of types known to be adapted to or confined to the warm tem- 

 peratures or moist climatic conditions of the present day, types that though 

 now extinct once lived in association with other types of ascertained tropical 

 or humid habitats, and types whose descendants or nearest surviving relatives 

 are characteristic of warm climates. Examples are cycadalean types in Car- 

 boniferous, Triassic, Jurassic, Cretaceous, and, finally, in the Oligocene, in 

 association, since the Trias, with living tropical and subtropical genera or 

 families; the presence of tree ferns in nearly all periods of coal formation; 

 palms, cinnamon trees, climbing ferns, and many other tropical or subtropical 

 types in the Upper Cretaceous; and breadfruit-trees, etcetera, in the Lower 

 Tertiary. 



"5. Structures of the plants themselves. Features showing rapidity of 

 growth — ^that is, abundant rainfall, mild or warm temperatures, etcetera — 

 conditions favorable to rapid growth : 



"(a) A^ery large size of the cells, many with thin walls, and large inter- 

 cellular spaces, indicating rapid growth and abundant moisture, noticeable in 

 the woods found in and with most coal. 



"(Z)) Large size of fronds and leaves, indicating conditions favorable to 

 growth and, at present, characteristic of moist tropical habitats. 



"(c) Frequency of lacinate or much-dissected, drooping fronds and pendent 

 branches or twigs seemingly adapted to facilitate the run-off of rain, and pro- 

 tection of the stomata in grooves on the under sides of many leaves, as in the 

 Lepidophytes of the Carboniferous. 



"(rf) Smoothness of bark, which is often thick, pointing toward warm, humid 

 swamps. 



"(e) Absence of growth rings in the woods of the older coal formations, 

 showing climatic conditions favorable to practically uninterrupted growth, 

 and the absence of long dry seasons or winter frost. Such absence of rings, 

 when noted in all the associated types, plainly shows the approximation to 

 equability of climate. 



"(/) Wide occurrence in the Paleozoic coal fields of heterospory, requiring 

 prevalent swamp conditions ; and the occurrence of delayed fertilization and 

 of devices for seed flotation. 



