^38 JOURNAL OF THE ARNOLD AUROUETIM [vol. i 



times, pro])a])Iy having had an unhiterrupted devek)pnieut from forests of 

 similar floristic cliaracter in the early Tertiary period, 



A great amount of evidence supports this belief; but it may be sufficient 

 here to mention that fossil floras as early as the Lignitic stage of the Eocene, 

 found in some parts of the region, furnish records of a forest even then com- 

 posed of genera mainly identical with and of many sj)ccies similar to those of 

 the present. Comprising such familar trees as Cypress, AVillows, Hickories, 

 Oaks, Elms and Magnolias conditions under which tliis early forest flour- 

 ished nuist have been in many respects similar to those that obtain there 

 today. 



Strong internal evidence of its antiquity is also furnished by the fully 

 developed state of the forest of the coastal plain, the variety and highly 

 specialized character of its components, the large percentage of endemic 

 species amongst them and the complete manner in which it has occupied 



the region and each si)ecies its i)articular station. Changes in the composi- 

 tion of the flora through the elimination of sj)ecies and tlu^ development or 

 introduction of new ones have, of course, being going on through all this 

 Vast time; and while in the main preser\M*ng its continuity the boundaries of 

 the forest have been constantly shifting: advancing upon one front over 

 new-made lands or in response to some favorable modification in geography 

 or clin:ate; in another quarter, perhaps, steadily retreating for centuries be- 

 fore adversely clianging conditions or by the quick destruction of large sec- 

 tions by some sudden vicissitude of nature. 



It is probable that gradual changes on a large scale in the boundaries of 

 the great southern forest are going on at present in several areas. An exam- 

 ination of its depleted ranks and the shattered front it presents along the 

 line where it disapj)cars in eastern Texas and of the conditions that confront 

 it there indicate that it has for a long time been losing ground in that direc- 

 tion. The fact that extreme eastern Texas is a region of high liumidity, 

 receiving an annual rainfall of from fifty to sixty inches, while on the w estcrn 



boundary of the state the precipitation is scarcely more than ten inclies, 

 sufficiently accounts for the disappearance westward one by one of the 

 moisture-loving forest species. But there is reason to l)elieve that condi- 

 tions have not always been thus and that at no very distant j)eriod as meas- 

 ured by geology, though immensely remote by historic standards, the great 

 forest extended unbrokenly along the western portion of tlie Gulf coast, up 

 the Kio Grande valley and across the Edwards Plateau, perhaps almost to 

 the base of the Rocky Mountains. Climatic changes resulting from the last 

 mountain-making movements and elevation of land in that region marked 

 the western portion of this forest for destruction and gradually {>iished it 

 back to its present limits, allowing the semi-desert flora of the Southwest 

 and of northern Mexico to invade and establish itself over most of tlie area. 

 Here and there throughout the region vestiges of the old flora may still be 

 found clinging to small areas where more favorable soil or moisture condi- 

 tions have enabled them to survive. Amongst many exam])les of this class 

 are the groves of Hickory, Post Oak and Black Jack occupying areas of 



