MIGRATION 61 



nance of the climax itself is almost exclusively a matter of propaga- 

 tion, supplemented, in a small degree, by regeneration. 



Leaving aside the consideration of movement in water, which is 

 chiefly due to current and wave, mass migration, as contrasted with 

 the transport of individual migrules, regularly takes place in two 

 fashions. Locally, it operates upon minor communities, utilizing prop- 

 agules in the case of the climax and disseminules in the colonization 

 of bare areas. Regionally, it becomes significant only under the 

 impulse of climatic changes, but the actual advance is due to the slow 

 and repeated movement of dominants and subdominants through both 

 these methods. The migration of ruderals and cultivated species has 

 likewise a mass effect in a large degree, in spite of the fact that trans- 

 port of weeds by man is unintentional. In all these cases, however, 

 the final value of migration as a process is determined by the success 

 attained by ecesis. 



The Migration of Animals. Migration proper in animals does not 

 differ from that of plants. Sessile and sedentary species, as well as 

 animals of limited activity in water, commonly possess a life-history 

 form that may be carried some distance by currents. On land, small 

 animals, especially insects, spiders, and some mollusks, may be moved 

 into new territory almost as readily as the seeds of plants. The num- 

 ber of these carried out of their homes by wind is evidenced by the 

 line of living drift found about Lake INIichigan after a storm of short 

 duration. However, the habits of the species are regularly so little 

 suited to life on the beach as to bespeak the rarity of establishment. 

 Nevertheless, the effect of such events in populating denuded areas 

 where animals precede plants cannot be neglected (Smith, 1928), nor 

 can the process be overlooked where climatic changes have favored 

 invaders into established communities. For example, species charac- 

 teristic of dry oak-hickory woods appeared in moist oak-maple forests 

 following the dry season of 1930, only to disappear soon after. 



The random wanderings of the larger motile animals out of their 

 usual range is of no more significance than the movement of the wind- 

 blown insects, since they regularly return and no actual shift of home 

 occurs. Other local or recurrent wanderings occur, but even the more 

 important and definite ones are subordinate to the mass movements 

 involved in biotic migration. 



Types of Migration. These may be grouped as: (1) recurrent 

 migrations, divisible into (a) annual, (b) seasonal, (c) metamorphic, 

 (d) diurnal; or (2) non-recurrent migrations, divisible into (a) exten- 

 sion of range, (6) local movements within the home area. 



The annual and seasonal migrations are characteristic of many 



