CHAPTER XI 



CONDITIONS GOVERNING THE EVOLUTION AND DISTRI- 

 BUTION OF TERTIARY FAUNAS' 



W. H. BALL 

 U. S. National Museum, Washington D. C. 



The subject allotted me being "The Conditions Governing the 

 Evolution and Distribution of Tertiary Faunas," I may begin by 

 stating certain propositions which, for the purposes of this discourse, 

 may be assumed as axiomatic. 



1. A fauna is an assemblage of organic species populating a given 

 area at one and the same epoch, and — allowances being made for 

 the preferences of such minor groups as carnivorous, phytophagous, 

 littoral, benthal, petricoline, and limicoline animals — having for the 

 most part identical geographical distribution. 



2. We may regard it as indisputable that the properties of the 

 environment shown to influence a living fauna, or to control its distri- 

 bution, were capable in Tertiary^ times of exerting an analogous 

 intluence on faunas now known chiefly by their fossil remains; and, 

 conversely, if in a fossil fauna we are able to trace certain definite 

 features, which in a living assembly would result from a particular 

 environment, we are justified in concluding that the fossil fauna in 

 question was, when living, subject to the action of an analogous 

 environment. 



To illustrate this second proposition it may be said that if fig trees 

 can now flourish and reproduce their species only in regions having 

 a mean minimum temperature of thirty degrees Fahrenheit, and a 

 summer mean temperature of not less than sixty degrees; and, 

 secondly, if we find in the Tertiary leaf-beds of Greenland and 

 Spitsbergen indications of groves of fig trees having flourished there 

 in the Oligocene epoch; then we are likewise justified in assuming 



1 Published by permission of the Director of the U. S. Geological Survey. 



2 The author realizes that these factors may not be entirely applicable to the faunas 

 of pre-Tertiary epochs. 



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