RELATIONSHIPS OF METASPERMAE. 597 



themselves of greater or of less magnitude. This effect may 

 be either mediate through the modification of climate or imme- 

 diate by the alteration of topography. And still again, the 

 countless variations in those conditions which, from their com- 

 plexity, are given the name of biological, have marked and 

 ample influence upon the general rate of progression. The 

 entrance and acclimatisation of some alien species of plant or 

 animal, the activity of man in burning or felling the forest and 

 in tilling the meadow-land or prairie, the movement of herds 

 of ruminating animals, such as the now almost extinct bison, 

 the flight of migrating birds, invasions of destructive insects 

 or of parasitic fungi — all these and many other kindred phe- 

 nomena may and do affect the movement of the line of tension, 

 by distributing seeds, destroying rival plants, introducing new 

 competitors and altering the dynamic equilibrium either gener- 

 ally or locally, and either continuously or discontinuously. 



Influence of equatorial pressure on habitat. The general 

 existence of equatorial pressure, of tension- lines and the laws 

 of the x^rogression of the tension-line, having now been noted 

 briefly, it remains to observe what is the influence of equatorial 

 pressure on the selection of habitats. Under the relentless 

 ejection of the weaker plants from the more favorable locali- 

 ties, and the increasing solidarity of the stronger plants in 

 characteristic formations, it is apparent that greater and 

 greater specialisation of form and physiology, together with 

 increasing specialisation of habitat, must arise. It is therefore 

 interesting to observe that the highly special habitat is com- 

 monly occupied by the highly specialised plant. The epiphy- 

 tic orchids which have accommodated themselves to a condi- 

 tion considerably removed from the original aquatic condition 

 of plants, are themselves members of the highest family of the 

 monocotyledons. The cacti of the arid regions, the dodders 

 that entwine themselves about the stalks of other plants, the 

 bladderwort which floats upon the surface of stagnant pools 

 and feeds itself with minute Crustacea that it has learned to 

 capture in its bladdery weirs, are all plants high in their re- 

 spective divisions. On the other hand the cat- tail ( Tijpha 

 latifolia), one of the lower plants of its division, is less special- 

 ised in habitat The least specialised habitat, the aquatic, is 

 peculiarly the region of the lower groups of the Metaspermae. 

 A most general result then, of the equatorial pressure is seen 

 in the specialisation of habitats. This is a result of the com- 

 petition following the ejection of the weaker. 



