ADVANCING AND RETREATING MIGRATIONS 47 



Significance of the Margin of Range. — The margin of the range 

 of a species, then, does not always represent the boundary of the ter- 

 ritory in which the species can or will live under present conditions, 

 but merely the distance which it has travelled so far in its march to 

 this goal. Instances are not lacking, of course, in which this migration 

 has been completed and further extensions or retractions of the range 

 wait upon future climatic changes. As one illustration, the distribu- 

 tion of Pinus ponderosa along the eastern foothills of the Rocky 

 Mountains may be cited, where it has been clearly shown 15 that the 

 species is now at the margin of its potential range. On a smaller 

 scale, a sharply marked [boundary between two associations indicates 

 completed migration in that restricted area of the species on both sides 

 of the boundary to the limits permitted by the present environment. 



Advancing and Retreating Migrations in the Middle West. — 

 Both advancing and retreating migrations are now in progress in the 

 states of the Middle West. In most cases these movements are too 

 slow to come under direct observation, and reported extensions of 

 ranges are generally due to incomplete or faulty early observations. 

 Still the rapid spread northward of Prosopis glandulosa from Texas 

 across Oklahoma into Kansas, 16 the westward migration of trees along 

 the rivers of Nebraska, 14 ' 17 and the recent discovery of young trees 

 of Quercus palustris in southeastern Nebraska 18 may be cited as in- 

 stances of visible migration. In general, migration is shown chiefly 

 by the role of the species in succession. If, at or near the edge of its 

 range, it participates in succession as an invading species, it is advanc- 

 ing; if it io not able to hold its own in competition, if the associations 

 of which it is a member are regularly succeeded by others of which 

 it is not a part, a retreating migration is indicated. This is illustrated 

 in Michigan and Wisconsin by the regular succession of coniferous asso- 

 ciations by deciduous forests, indicating the retreat of the conifers and 

 the advance of the deciduous trees. 19 



Effects of Changes in Physiological Requirements. — There is 

 no reason why changes in the physiological requirements of a species 



ij Dodds, Gideon S., Francis Ramaley, & W. W. Robbins. Studies in mesa and 

 foothill vegetation, I. Univ. Colorado Studies 6: no. 1, 11^9. 1908. 



is Bray, William L. Distribution and adaptation of the vegetation of Texas. Bull. 

 Univ. Texas 82. 1906. 



it Bessey, C. E. Are the trees advancing or retreating upon the Nebraska plains? 

 Science 1L, 10: 768-770. 1899. 



is Pool, Eaymond J. Pin oak in Nebraska. Torreya 20 : 50-52, 1920. 



19 Whitford, H. N. The genetic development of the forests of northern Michigan: 

 a study in physiographic ecology. Bot. Gaz. 31: 289-325. 1901. 



