MIGRATION AND SUCCESSION 43 



crops of seeds or other reproductive bodies and these are dispersed 

 thoroughly over the surrounding territory to an extent dependent upon 

 their structure and their environment. At the margin of the present 

 range of a species, whether that margin is the boundary of an associa- 

 tion in which it occurs or the general margin of its whole range, seeds 

 are annually dispersed in all directions, inward through its present 

 range and outward and beyond it as well. Examples of this are well 

 known and need not be cited. Except in comparatively infrequent 

 cases, where the ability to produce seeds or other reproductive bodies 

 is lost, the potentiality of continuous and extensive migration is con- 

 stantly present. The slow rate of actual normal migration is not due 

 to inefficiency of the migratory devices of the plant, nor, except as 

 mentioned above, to deficient seed production, but to the inhibiting 

 effect of environmental factors in the area beyond the present distri- 

 bution of the species. This is particularly true toward the center of 

 the range. Every swamp is annually and abundantly planted with 

 seeds of upland species, while the swamp plants are simultaneously 

 distributed over the upland, and in neither case do any of them grow, 

 except under the most unusual circumstances. 9 Even at the margin 

 of the specific range, where reduced or intermittent seed production 

 might be expected, Griggs has shown that many species are able to 

 propagate themselves freely. 10 



There is for each species a general limit of distance over which its 

 seeds may be dispersed. The number of seeds arriving on any area 

 diminishes with increasing distance from the parent plant ; in fact, for 

 small distances, the number seems to vary inversely as the square of 

 the distance. Since only a small portion of the seeds germinate and 

 grow to maturity, most young plants of the second generation are located 

 relatively close to the parent. 



Migration and Succession. — Actual migration, therefore, requires 

 a change in the existing environment for its inception. (The abnormal 

 type of migration, due to the acquisition of new means of dispersal 

 by which previous environmental barriers are crossed, is mentioned in 

 a later paragraph.) A migratory advance postulates the development 

 of individuals in territory not previously occupied and requires an 

 environmental change of sufficient extent not merely to permit the 

 growth of the migrants, but to permit it against the competition of 

 the previous inhabitants. A migratory retreat requires a detrimental 



» Brewster, William. Occurrence of the skunk cabbage in an unusual place. 

 Khoflora 11: 63, 64. 1909. 



io Griggs, Robert F. Observations on the behavior of some species at the edges 

 of their ranges. Bull. Torrey Club 41 : 25-49. 1914. 



