50 H. A. GLEASON VEGETATIONAL HISTORY OF MIDDLE WEST 



Whole floras migrate together, the hardiest and most mobile species 

 first, the others in their train. They establish successional series in 

 the region which they enter, and build up new associations in the new 

 territory. These are analogous to the associations of their original 

 range, but differ in the absence of the slower moving species and in the 

 presence of laggards from the retreating flora. 



Just as portions of a single species may be isolated during its migra- 

 tions, so may areas of one type of vegetation be completely surrounded 

 by an advancing flora and left isolated from the main body. These 

 are known as relic colonies. The cause of such isolation is the failure, 

 up to the present time, of sufficient environmental change to cause their 

 extinction, or, in ecological terms, it is the relative slowness of the suc- 

 cession in certain habitats unfavorable to most of the advancing species. 

 These habitats remain with their original vegetation upon them, while 

 elsewhere it is displaced. Such relic colonies are of common occurrence 

 wherever migrations are recent. Every movement of vegetation has cer- 

 tainly left relic colonies behind it, but their perpetuation to the present 

 time depends absolutely on the continuation of an environment not 

 only favorable to the relic species but distinctly unfavorable to the 

 invaders. ! 



Adjustments in Distribution. — The distribution of a species and 

 the distribution of vegetation must both be regarded as subject to con- 

 tinual adjustment. Boundaries extend and contract, associations 

 advance and retreat with every geographical fluctuation of the environ- 

 ment. Even within the limit of a single association, every minor en- 

 vironmental variation, whether geographical or seasonal, is marked 

 by the entrance or disappearance of some species, or by changes in the 

 numerical proportion of the others. This may account for the condi- 

 tions recently reported on Mt. Marcy, 22 where twenty-one species 

 formerly reported by Peck have apparently disappeared and seven. 

 " rather conspicuous plants, which such a careful botanist could scarcely 

 have missed, have apparently come into the flora." 



General Results of Continued Migrations. — Obviously the 

 general result of continued migrations is the mingling of floras of 

 diverse origin, while great climatic changes or important geological 

 episodes tend toward the segregation of floras and the consequent 

 initiation of new migrations. Regions affected by Pleistocene glacia- 



22 Adams, C. C, Geo. P. Burns, T. L. Hankinson, Barrington Moore, & Norman 

 Taylor. Plants and animals of Mount Marcy, New York. Ecology 1 : 71-94, 

 204-233, 274-288. 1920. 



