48 H. A. GLEASON VEGETATIONAL HISTORY OF MIDDLE WEST 



might not have the same effect as a change of environment in causing 

 migration. It is well known that plant breeders have developed new 

 races of cultivated plants which thrive in climates different from that 

 of the original home of the species. Possibly the rapid spread of 

 American Opuntias in parts of Australia may be explained in this 

 way. Under natural conditions, such changes in physiological require- 

 ments are usually correlated with changes in structure, and the latter 

 are used as a basis for taxonomic differentiation. Whenever such 

 evolutionary changes appear, new areas may be opened to the new 

 species and eventually occupied by them. So such genera as Eupa- 

 iorium and Vernonia, originating probably in northern South America, 

 have migrated northward into the cold-temperate zone, as evolution has 

 changed their physiological requirements and morphological structure. 

 So have bromeliads migrated away from the humid tropical forests, 

 entered the deserts of southern Mexico, and established themselves as 

 epiphytes on the cacti ; 20 so have the cacti migrated out of the desert 

 region, where they probably originated, and established themselves 

 as epiphytes in jthe tropical forest. 



Migrations as Related to Continuity of Suitable Habitats. — 

 The range of every species is discontinuous. Throughout its whole area 

 only particular habitats are occupied, leaving others to different species 

 and associations. In each habitat, the space is shared with other 

 associated species. These phenomena are 'in general of geological 

 rather than geographical interest, but are still of importance in their 

 relation to the direction of migration. Within the habitat, so far as 

 the association is continuous, even the least mobile species may migrate 

 freely and ultimately attain a uniform distribution. 11 But from one 

 habitat to another, the means of dispersal must be sufficient to carry 

 the species across the intervening gap, which becomes a barrier to the 

 less mobile forms. During the long course of time, accidents of dis- 

 persal may carry many species across a barrier, but accidents are 

 probably not as efficient as frequently supposed. Mink Grove and 

 Lynn Grove, two isolated areas of forest in Champaign County, Illinois, 

 had large trees in them when first observed by white men nearly a 

 century ago, and their age is doubtless 150 years at a minimum. Al- 

 though these groves lie but a few miles from the nearest strips of forest 

 along streams, in which species of Hicoria and Quercas are abundant, 

 not an individual of these genera has reached them. Extensive migra- 

 tions of plants, proceeding at a relatively rapid rate, therefore take 



20MacDougal, D. T. Botanical features of North American deserts. Carnegie 

 Inst. Washington, Publ. 99. 1908. 



