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THE AMERICAN NATURALIST [Vol. XLVIH 



humidity we get a very close correspondence between distribution 

 and climatic factors. Schimper 10 has brought together a great 

 deal of evidence which indicates that, as far as plants are con- 

 cerned, even the major divisions of the world's surface into arctic, 

 temperate and tropical are fundamentally a question of the de- 

 mand for and supply of water. 



Furthermore, if recent climatic changes have an effect upon 

 the origin of new characters and the distribution of the organisms 

 possessing certain characters, humidity is deserving of more 

 attention than temperature, since practically the only evidence 

 we have of such changes concerns humidity. 



It should not be forgotten that even aquatic organisms are 

 subject to what amounts to changes in humidity. Peat bog plants 

 take on many characteristics of a desert flora, although their 

 roots are covered with water. It is water, however, which is 

 not easily available, because of the chemicals which it carries. 

 It is water which is physiologically dry. 



Finally, the great amount of work which has been done upon 

 artificial parthenogenesis and related subjects is, in a way, a 

 study of the influence of environmental factors. The obvious 

 factors concerned have usually been various chemicals but at 

 foundation humidity, in a broad sense, the addition or withdrawal 

 of water by osmosis seems to be a factor of prime importance. 



Frank E. Lutz 



American Museum of Natural History 



10 "Plant Geography upon a Physiological Basis," translated by W. E. 

 Fischer. Oxford, 1903. 



