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SCIENCE. 



[N. S. Vol. XII. No. 309. 



category, including such woody plants as 

 Decumaria, Itea, Callicarpa, Oxydendrum, 

 Aralia spinosa, Vitis rotundifolia, etc. A 

 similar case is the presence of Azalia viscosa, 

 an essentially Coastal Plain species, here 

 and there in the mountains along with its 

 mountain analogue, A. arborescens. Leucothoe 

 racemosa, abundant in the swamps of the 

 seaboard, is also found occasionally along 

 highland streams, while a closely related 

 and very similar species, L. reeurva, is much 

 more abundant in the mountains, to which 

 it is confined. These are cases where the 

 differentiation in distribution of correspond- 

 ing forms, one in the Coastal Plain, and 

 another in the Appalachian region, is either 

 incomplete or has not taken place at all. 

 But as no fact in biology is better known 

 than the capacity of some species to endure 

 a wide range of physical conditions, while 

 others are fatally sensitive to compara- 

 tively slight differences of environment, 

 this difficulty is not an insuperable one. 



The initial appearance in the mountains 

 of species of the first category, i. e., those of 

 presumably neotropical origin, was probably 

 somewhat subsequent to the return thither 

 of the Miocene Boreal forms of the second 

 category, for most of the former require 

 decidedly higher temperatures than many 

 of the latter. But we know little of the 

 history of such groups as are chiefly repre- 

 sented in this category and which make up 

 a large part of the modern tropical Amer- 

 ican flora, i. e. , the above mentioned tribes 

 of Graminese, Leguminosse and Compositse. 

 Hence we must content ourselves with as- 

 suming that these species did not exist in 

 the Appalachian region prior to recent geo- 

 logic time, and that they constitute the most 

 modern element of its flora. 



It is more than probable that the hy- 

 pothesis just outlined is very incomplete as 

 to details and will be found not to account 

 for all the phenomena. Instead of the com- 

 paratively simple progression of events 



which it premises, the fact is pretty well 

 established that there was more than one 

 advance and recession of the Ice Sheet, 

 and that the mutations of the flora have 

 been correspondingly intricate. But of the 

 complex of factors which have been at work 

 since the middle of the Tertiary in giving 

 to this flora its present distribution, we 

 know far too little to permit the elaboration 

 of a more comprehensive theory. Until 

 we possess a much larger body of paleon- 

 tological evidence, and a better understand- 

 ing of past climatic conditions, we must be 

 content with some such working hypoth- 

 esis. 



When we come to inquire into the con- 

 ditions of climate and of soil which permit 

 the actual existence of numerous Lower 

 Austral forms in juxtaposition to a Transi- 

 tion and even Canadian fioi-a, we enter 

 upon an investigation that is within the 

 domain of exact research. Here we are 

 dealing with things tangible, which can to 

 some extent be weighed and measured. 



First let us compare the climate of the 

 Appalachian Eegion in the Southern States 

 with that which prevails under the same 

 latitude in the Austro-riparian area, direct- 

 ing our attention to the factors of tempera- 

 ture which have the largest effect in de- 

 termining the zonal distribution of organ- 

 isms. These are believed to be : (1) the 

 normal number of days during the year 

 which possess a temperature above 6° C. 

 (43° F.); (2) the normal sum total of tem- 

 peratures above 6° C. daring the period 

 thus defined ; * and (3) the normal mean of 

 the six consecutive hottest weeks. f In the 

 following table data are given for four sta- 

 tions in the mountain region and for two of 



*The factor which is believed to fix the northern 

 and upper limit of the great life zones. See Mer- 

 riam in Nai. Geogr. 3Iag., 6 : 229-238, 1894. Also 

 Life Zones and Crop Zones, Bull. Div. Biol. Survey, 

 U. S. Dept. Agric, 10 : 54, 1898. 



t The factor taken as determining the southern and 

 lower limit of the zones, Merriam, 1. c. 



