No. 624] MIGRATION A FACTOR IN EVOLUTION 63 



5. Biospheric Agencies in Transportation and 

 Migration 



(a) Plants.— The relatively sedentary and rooted con- 

 dition of plants caused Cope to aptly call them "earth 

 parasites." With this stable habit and the unstable en- 

 vironment, rooted plants have been forced to develop a 

 line of fracture, as it were, between themselves and the 

 environment, which has permitted them, by their excep- 

 tional powers of dispersal, to spread rather freely at 

 some stage, and to thus scatter over much of the avail- 

 able surface of the earth. As far as the actual move- 

 ments of plants are concerned, unaided by winds, wat- 

 ers, and animals, but solely by growth and similar move- 

 ments, plants have probably had but relatively small 

 influence upon animal transportation, although second- 

 arily, by the spreading of vegetation and the changed 

 conditions which this causes, they have permitted ex- 

 tensive transportation and migration of animals. The 

 specific gravity of wood, its buoyancy in water, and the 

 various sails, vanes, and structures which favor wind 

 dispersal, and all the hooks and claws which help make 

 various kinds of burrs, and the edible fruits which animals 

 devour, all combine to favor transportation by wind, 

 water, animals, and other active agencies. By these 

 means, animals living within, or on such transported 

 parts, may be buoyed and transported by waves, currents 

 of water or air, and be carried by animals for long dis- 

 tances, and into new localities and conditions. 



(b) Animal Migration.— The movements of animals 

 which take them from one locality to another are exceed- 

 ingly diverse. They vary not only with the character of 

 the animal, at different stages in its life history both 

 structurally and functionally, and also under varied en- 

 vironmental conditions. The fixed and sessile animals 

 more nearly approadi the conditions f()nn<l commonly 

 among plants, l^ut ninoiiu- nnimals tliis lia1>it is contiiied 

 mainly to aciuatic animals, that is. to animals living- in a 

 mobile medium, which transports them at some stage. 



