INTRODUCTION. 



through the great extent of the plateau. Therefore, from its peculiar 

 position and character, thorough investigation of its native plants ought 

 to throw some light on questions of the geographical distribution of 

 plants in post-glacial times, and the derivation of local floras in the East- 

 ern United States. Final, or indeed valuable conclusions on these ques- 

 tions, we are not at present prepared to give, but shall present those we 

 think our investigations thus far will warrant. 



To ascertain the affinity of an insular flora, or that of a continental 

 basin, is an easier task than to set forth the exact truth concerning the 

 genealogy of a small flora, one belonging to a series of contiguous hydro- 

 graphic basins. Our method of examining thoroughly the distribution of 

 plants over a small hydrographic area, and connecting this with a similar 

 work in a neighboring area, also limited by natural, not artificial, bound- 

 aries, we believe to be thoroughly scientific. Moreover, we believe that 

 induction based on collated evidence of this character will furnish the 

 only conclusions in the future on the evolution of the floras of continental 

 basins as we now find them, which will be acceptable to the scientific 

 mind. The main facts of geographical distribution are within our reach, 

 but the evolution of the present distribution, excepting in the case of 

 islands or isolated mountains, is a question almost wholly unexplored. 



The affinities of a small flora are to some extent shown by the 

 commonest wild plants and by the rarest. The former are present in 

 related floras because inter-distributing has been easy and the conditions 

 of soil and climate particularly favorable. Rare plants may occur for a 

 variety of reasons ; but if it is because the region where we find them 

 has so changed in the conditions favorable to their existence that they 

 are being gradually exterminated, then indeed we have in them a pre- 

 cious title-deed, without which we should not be in possession of a series 

 of facts or suggestions, most valuable from the point of view of science. 

 Again, if conditions exist in our region, allowing the rare occurence of 

 plants which are common much farther north or south, while such spe- 

 cies do not appear in contiguous regions, the exact nature of those con- 

 ditions becomes interesting to us, and usually not difficult to ascertain. 



There is another method of determining the relationship of a small 

 flora, however, which seems to us of more value than any other, although 

 its successful and judicious application requires experience. If we find 

 that a small number of species, of coast plants for example, extend to 

 this region but no further west, while a greater number of southern spe- 

 cies find their northern limit here, we conclude the flora has greater 

 affinities with the South than with the Atlantic slope. If we find this 



