40 MEMOIRS NATIONAL ACADEMY OF SCIENCES. [voL.xn 



THE EVOLUTION OF THE GROSS ENVIRONMENT. 



To study life we must consider three things: First, the orderly sequence of external nature; second, the living 

 organism and the changes which take place in it; and, third, that continuous adjustment between the two sets of 

 phenomena which constitutes life. — W. K. Brooks, 1899. 



1. INTRODUCTORY. 



The preceding part of this paper has been devoted to a detailed consideration of the facts 

 of the variation and distribution of the forms of lo, considered mainly from the standpoint of 

 masses or as populations. A sifting process has been carried on in order to secure fairly homo- 

 geneous imits and populations which represent the average status of lo in the various parts of 

 its geographic range. These quantitative studies aid in giving a fairly precise and uniform 

 method of describing the local differences found. Attention has been given primarily to this 

 descriptive aspect, theoretical considerations and interpretations having been left in the back- 

 groimd because of the greater opportunity for the correlation of their general relationships in 

 the topical treatment which will follow. 



We wish now to turn to another series of facts, fundamental to our problem, but whose 

 intimate relation may not be apparent at first glance, the development or genesis of the environ- 

 ment in which lo has developed. The intimate character of this relation is frequently neglected 

 or even ignored in zoological studies, and yet it is an essential part of such a problem as is here 

 under consideration. This relation has been very clearl}^ expressed by Brooks ('99, p. 54), as 

 follows : 



The physical sciences deal with the external world, and in the laboratory we study the structure and activities of 

 organisms by very similar methods; but if we stop here, neglecting the relation of the living being to its environment, 

 our study is not biology or the science of life. 



In this discussion we will consider the "orderly sequence of external nature," or, in other 

 words, the orderly sequence followed in the development of the gross environment, so that we 

 may see how this factor has been able to influence lo. 



To make this point of view as concrete as possible has been one of the principal aims of 

 this section. To be sure the results of such attempts to harmonize the "internal" and "envi- 

 ronmental" relations of lo are not entirely satisfactory in many respects; and yet no real 

 advance can be made without squarely facing the problems and attempting to analyze and 

 formulate them. It is considered that a knowledge of the development and structure of the 

 environment is as essential in our problem as is the development and structure of the animals 

 themselves. Attention is therefore given to those conditions and forces which appear to be 

 the dominant environmental factors, so that we may see how the general principles involved 

 are relatively few and orderly rather than chaotic and innumerable, as they may appear upon 

 superficial examination. 



Considered in detail, the complexity of the problem is so great that it is comparatively 

 easy to overlook the general principles. It is mainly for this reason that much emphasis must 

 be placed upon these dominant environmental featm-es and their sequential relations. 



The evolution of the Southern Appalachian province, and particularly the region of the 

 upper Tennessee drainage, in which lo has evolved, includes three classes of variables: First, 

 those geologic influences or processes which have caused the uplifting and depression of the 

 land surface relative to sea level; second, those processes of degradation of the land which tend 

 to reduce it again to sea level; and third, the composition and structure of the rocks. The 

 complexity of the evolution of the drainage of this region is due to the interrelations of these 

 three classes of influences. Since the region now occupied by the Appalachian Valley became 

 dry land, at the close of the Carboniferous, its history, in harmony with the above classes of 

 variables — crustal movements, degradation, and rock structure — ^has probably been, in brief, 

 as follows: According to Hayes and Campbell, it was elevated, folded, and truncated by erosion 

 to a final peneplain at the close of the Cretaceous; then was again upHfted and again partly 

 reduced by erosion during the Tertiary, and still again uplifted to form the present cycle; or, 

 according to Keith ('96, p. 524), there have been four periods of "approximate reduction" to a 

 peneplain. 



