46 H. A. GLEASON VEGETATIONAL HISTORY OF MIDDLE WEST 



there is a continuous movement of the more mobile forest species into 

 the land originally occupied by prairies. 



The time factor naturally begins with the origin of the species con- 

 cerned. Those of relatively recent origin have had little time in 

 which either to extend or to contract their original ranges. Since 

 major environmental changes are usually slow in their accomplish- 

 ment, recent species have in general had also relatively little oppor- 

 tunity to use their migratory abilities. Such species are therefore 

 restricted more closely to the site of their origin. 



It is probably true, however, that many of our species have not 

 had a single point or even a limited area of origin. The bulk of later 

 Tertiary plants, as far as palaeontological evidence indicates, are of 

 genera still existing, and many of the comparatively few known species 

 of Pleistocene plants are either identical with or closely similar to 

 existing species. Such evidence deals especially with woody plants, 

 but what is true of them is doubtless true of herbaceous plants as well. 

 The distinction between such Pleistocene plants and their modern 

 representatives may depend largely on a break in the record, on a 

 period from which fossils are lacking. Probably if a complete series 

 of specimens were at hand, showing comprehensively the maples of the 

 eastern states, for example, from the Pliocene to the present time, it 

 would be seen that some of the earlier forms are absolutely continuous 

 with our present species and that the slight morphological distinctions 

 between them are only the result of continuous slow variation through- 

 out the centuries. According to this view, many modern species had 

 no localized origin and are not the offshoot of any parent, but represent 

 the mass development of a species, which, under our present taxonornic 

 ideas, came to a stop at the beginning of ,a break in our geological 

 record of it and reappeared as a new species at the beginning of our 

 next experience with it. During the long history of such a species, 

 most of which is unknown to us, it may have migrated repeatedly in 

 various directions, occupied larger or smaller areas, been separated 

 into disjunct regions and again united, or one part of its population 

 exterminated, and its range as we see it today represents merely the 

 present condition of this long development. We are never sure of 

 the prehistoric stages in this history, through the imperfections in the 

 fossil record, but in certain cases we can arrive at some idea about 

 them by circumstantial evidence. 



Such a consideration does not account for the multiplication of 

 species. Whether that is caused by mutation, natural selection, or 

 hybridization, it seems certain that their perpetuation to the present 

 time has depended greatly upon their ability to migrate as changes 

 of environment demanded. 



