THE ELEMENTS OF A PALEOGEOGRAPHIC PROBLEM 41 



as the problem is approached from different angles. In a previous paper the 

 author has suggested a scheme which seemed the best for his purposes: 1 



Favorable, hospitable. Unfavorable, inhospitable. 



Active hospitality. Active inhospitality, antagonism. 



Passive hospitality. Passive inhospitality. 



Inorganic contacts may be classified under the same heads. Such con- 

 tacts will be with the atmosphere, hydrosphere, and lithosphere; they will 

 almost invariably take place, as Chamberlin has pointed out, at the surface 

 of one of these or in a narrow zone where two of the spheres meet. 



Contacts with the atmosphere will be both dynamic and static. The 

 dynamic contacts will be with all the movements of the air which in any 

 wise condition or effect the movements, life, or distribution of organisms. 

 Temperature and pressure are commonly static factors, but in so far as either 

 determines the movement of the air they must be reckoned as dynamic. 



The static contacts with the atmosphere are the temperature (constant, 

 annual average constant, annual average range, average seasonal variation, 

 etc.; if there is a progressive change in any of these it will be an effective 

 factor), the pressure, and the constitution (including the water-content or 

 humidity). 



Contacts with the hydrosphere will be essentially the same as with the 

 atmosphere, as it is a mobile sphere; but there must be added the factor 

 (largely static) of the change of state as the temperature fluctuates across 

 the point determining solidification or liquefaction. Movements in ice- 

 masses would afford the same kind of contacts as with the lithosphere. 

 The constitution of the water will afford a much greater variety of contacts 

 than that of the air, because the water contains large and various quantities 

 of material in solution, while the air is, so far as we know, a mechanical 

 mixture of gases and the variation of its constituents is within a limited range. 



Contacts with the lithosphere are predominantly static. Such qualities 

 as hardness, texture, chemical and mineral composition, water-content, 

 position and posture of rock layers and masses, depth of soil, etc., occur at 

 once as important factors in the environment. Sudden movements in the 

 lithosphere, as landslides, earthquakes, etc., are far too brief and localized to 

 affect more than a portion of one generation, but may be effective as a 

 dynamic factor in the extinction of a local fauna or flora. The slow move- 

 ments resulting in soil accumulation and denudation are certainly effective 

 in their influence on the evolution of a group, but are normally so slow that 

 the condition in any unit of time, even a unit of considerable duration, may 

 be regarded as fixed and the contacts will be static. 



One other classification of the environment is valuable in realizing the 

 effect upon life. An environment may be monotonous or diversified. 



1 Case, E. C., (Ecological Factors of Evolution, Bull. Wis. Soc. Nat. Hist., vol. 3, n.s., pp. 

 169-180, 1905. 



