November 30, 1900.] 



SCIENCE. 



839 



The difficulty of such an assumption is 

 increased by the fact that some of the forms 

 belonging to our second category have ap- 

 parently undergone little modification since 

 Pliocene times ; and this may well be true 

 of many of them vyhose past history is still 

 unknown. To the average mind, the alter- 

 native hypothesis, that of an extensive 

 migration of the less resistant species from 

 the mountains to the warmer lowlands, is 

 decidedly more thinkable. 



As the Ice Sheet began to recede, and the 

 climate of the Appalachian Region became 

 gradually milder, approaching its present 

 character, those species which had resided 

 in the Appalachian Region before the Pleis- 

 tocene, would have gradually returned 

 thither; but as the climate of to day is 

 probably considerably colder than that of 

 the Pliocene, it is to be presumed that this 

 floral element now occurs at a lower alti- 

 tude than that at which it flourished in 

 pre-Glacial times. It may be assumed, 

 furthermore, that the neo-tropical forms 

 which constitute our first category, then 

 began to make their way, for the first time, 

 into the Appalachian Region. 



To account for the presence to-day of 

 representative species of certain genera 

 (e. g., Stuartia, Fothergilla) in the moun- 

 tains and in the Coastal Plain, respectively, 

 it is conceivable that after the final retreat 

 of the great glacier, the increasing heat of 

 tbe lowlands induced in some individuals 



climatic conditions to which they have previously 

 been adapted. This point is well brought out by H. 

 TOn Ihering in a paper on ' Die neotropisohe Tropen- 

 gebeit und seine Geschichte ' (Engler Bot. Jahrb., 

 vol. 17, Beiblatt 42, 1893). It is easily conceivable, 

 for example, that vegetation as a whole has been ao-. 

 customing itself, during long ages, to gradually de- 

 creasing heat. But, in the case which we are here 

 considering, this objeotion cannot be allowed much 

 weight, as the climatic changes have been more or 

 less oscillatory, rather than progressive and have 

 taken place within a (geologically speaking) com- 

 paratively brief period. 



of a single ancestral species, which had 

 sought refuge there during the Ice Age, 

 changes of physiological constitution and of 

 structure which fitted them to endure a 

 warmer climate than that to which tbey 

 had previously been accustomed. Other in- 

 dividuals having gradually made their way 

 to higher elevations on the heels of the re- 

 treating Boreal flora, settled finally in the 

 valleys and on the lower slopes of the 

 mountains, where they have remained up 

 to the present day, perhaps with little varia- 

 tion from the Pliocene form. 



If we assume, on the other hand, that 

 forms contained in our list of representa- 

 tive species were enabled to survive the 

 Glacial Epoch without migrating, in toto, 

 from the Appalachian Region, an alterna- 

 tive hypothesis becomes possible. 



In that case it may be conceived that 

 while some individuals of each hypothetical 

 Pliocene ancestral species maintained them- 

 selves in well- sheltered situations and were 

 not forced to a change of abode, others 

 escaped the changing environment by a 

 gradual retreat into the warmer lowlands. 

 The individuals which remained in the 

 mountains were the direct ancestors of the 

 present Appalachian species ; while those 

 which migrated and later accustomed them- 

 selves in the Coastal Plain to the increas- 

 ing temperatures that ensued upon the close 

 of the Glacial Epoch, gave rise to the Aus- 

 tro-riparian species that attract our atten- 

 tion to-day because of their close resem- 

 blance to Appalachian forms.* 



It is true that this theory leaves unex- 

 plained the occurrence, both in mountains 

 and plain, of identical species of the second 



* It is not impossible that in some of these cases ol 

 representative species, differentiation of the allied 

 forms may have taken place before the advent of the 

 Glacial Epoch. But in most instances the relation- 

 ship is so extremely close that we need not assume for 

 them an older origin, especially as no other convenient 

 hypothesis offers to account for their present distribu- 

 tion. 



