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 progression 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 inits bladdery weirs, are all plants high in their re- 
spective divisions. On the other hand the cat-tail (Typha 
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. 
