ORGANIC CRITERIA 337 



The climatic factor of temperature is far less readily determined tluvu 

 that of humidity. Glacial conditions in their extreme development are 

 determinable in the character of the deposits. The oncoming of a glacial 

 climate is only to be argued from more obscure bits of evidence of a gen- 

 eral character, as progressive elevation and perhaps increasing aridity due 

 to the elevation. But here the location of the area with regard to barriers 

 to the prevailing wind must be considered. On the whole, this phase of 

 the evidence is not satisfactory. 



More positive evidence of the temperature may be gained from the 

 plant remains, if it is certain that they are in the region of their true 

 habitat. We have some definite information of the relation of the habit 

 of growth, shape of leaves, etcetera, to temperature which is usable. 



Dare we hope that some time a sufficiently definite determination of 

 the relation of climate to periods of volcanic activity will be made, so that 

 we may show some relation betAveen the animals buried in showers of 

 volcanic ash or inclosed in sediments of volcanic origin and their climatic 

 environment or the climatic environment of their immediate successors? 



Organic Criteria 



The inorganic record of past climate is only a portion of the story. It 

 has generally been assumed that plants are the best indicators of climatic 

 conditions, because of their relative immobility; but it is the nature of 

 plant material to float, and plant remains may be transported a great 

 distance, and even into regions of a radically different climate, before 

 they find their final resting place. Again, the occurrence of aquatic 

 plants in an arid region would be a surprise only to those who are un- 

 familiar with such regions. Each water-hole, seep, or spring is bordered 

 with water-loving plants, and the water is frequently filled with a lusty 

 growth. These are just the places where animals of an arid region would 

 congregate and where their remains would be found when they finally 

 mired down or were swept together after death by water currents. It 

 would be strange error to interpret the climatic environment of a bison 

 or a Pronghorn buck from the remains of an aquatic or semiaquatic xi^go- 

 •tation which grew in the mire of their final resting place. How easily 

 such an absurd mistake might be made is left to the imagination of every 

 collector. One check is fairly certain if the collector knows how to han- 

 dle his material; — the stomata of plants, either aquatic or terrestrial, 

 of an arid region are all well protected, for there is one common control 

 in the complex, and that is the drying winds. Even here the worker must 

 rememl)er that aquatic plants of stagnant water bodies have protected 

 stomata. 



