1921] PALMER, THE FOREST FLORA OF THE OZARK REGION 231 



plateau. At this line it was doubtless checked, affording time for many 

 of the less mobile species to come up and completely occupy the lowlands, 

 resulting at length in keen competition and a strong demand for further 

 expansion. In the meantime certain pioneer species had begun to push 

 up the alluvial valleys of the streams coming down from the highlands; 

 such trees as Willows, Cottonwood, Elms, Birch and various shrubs per- 

 haps first establishing themselves along the banks; while Sassafrass, 

 Persimmon, Dogwood, Plum and Haw-bushes began to appear along the 



margins of the more fertile prairies. A little later such hardy trees as 



some of the Oaks and Hickories, that in the southern area had been re- 

 stricted to the higher sandy ridges and thus inured to a considera})le 

 degree of drouth and soil sterility, began to creep up the rocky hillsides 

 and occupy the higher ground. Other forms followed as the developing 

 forest invited the coming of faunal agents that aided in their introduction, 

 or as other consequent or fortuitous opportunities arose, enabling them 

 to emerge from the overcrowded lowlands into the open if less opulent 

 plateau region. This movement has continued with great complexity 

 from that time, and is still in progress; but in certain Inaccessible spots 

 small remnants of the ancient floras are still holding out against the 



invaders. 



Much of the foregoing is, of course, hypothetical, but it is in accord 

 with many observed facts, and its fuller confirmation or elaboration de- 

 pends upon further investigations along various lines, especially in the 

 detailed study of the distribution of living and fossil species and in the 

 correlation of geologic and botanic evidence. The more significant 

 phenomena of the Ozark flora, as described above, may be summarized 



as follows: 



The identity of the great majority of the woody species found in the 

 Ozark region with those of the coastal plain forest and the gradual dis- 

 appearance of its less hardy and tolerent species towards the northwest 

 strongly point to the source from whence they have come and the direc- 

 tion in which they have travelled; the apparent lack of many endemic 

 species or of well marked varieties or geographic races amongst the woody 

 plants, in striking contrast to the much modified herbaceous floras both 

 of the northeast and southwest, is In accord with what would be expected 

 of a forest of comparatively recent introduction, and one which has never 

 been completely isolated from the main source of its origin; the peculiar 

 flora of the barrens and prairies, its obvious relation to that of the plains 

 and semi-arid regions of the Southwest, and the distribution of the local 

 areas where these plants occur indicate both the lines of the forest In- 

 vasion and the character of the flora that preceded it in the region. The 

 presence in the swampy lands along the eastern border, reaching nearly 

 as far north as the center of the Ozark plateau, of a rich ligneous flora of 

 markedly southern aflSnities certainly bears evidence of a climate decidedly 

 milder than that which prevails at present, and which could scarcely have 

 obtained subsequent to the complete elevation of the Ozark region and 



