596 METASPERMAE OF THE MINNESOTA VALLEY. 
habit of growth or habit of nutrition in the plants in question. 
Under these retarding influences the gradual development of 
monocarpic into polycarpic forms, of herbaceous into shrubby 
and of shrubby into arborescent types may be brought in evi- 
dence. As the distribution of the herb is more rapid than that 
of the shrub and the distribution of the shrub more rapid than 
that of the tree, any influences thatinduce the emergence of 
shrubby or arboreal characters may be deemed distinctly re- 
tarding in their general effect. That such a difference of 
mobility actually exists is derived from the testimony both of 
experience and of a priori reasoning. The adventive plants 
and the escaped plants in any region are always in large part 
herbaceous, because it is more easy for plants of small size and 
rapid maturation to gain a foothold than for plants of large 
size and slow maturation. But in the internal competition for 
light— the important energising force of plants—the emer- 
gence of the shrubby or arboreal character may be expected 
and precisely as it becomes more prominent—unless other 
modifications arise to maintain the general equilibrium— 
will the rate of distribution decrease. In general we see that 
the higher forms of archichlamydeous trees such as the linden, 
the maple, the walnut, are heavier seeded than the lower forms 
such as the willow or the poplar. The increased size of the 
seed is necessary to provide for the increased difficulties that 
surround the establishment of the seedling, So thus it is 
evident that the development of the arboreal type exerts a 
retarding influence upon distribution. 
Fluctuation in tensions. Fluctuations in equatorial pres- , 
sure may arise in several ways. Beside the general accelera- 
tion due to the increased extension of the central groups of 
species and formations and the general retardation due to the 
causes mentioned, there will arise fluctuations which may 
originate in widely diverse conditions. These conditions may 
be topographical, climatological, geological—in the widest 
sense—or biological. The erosive action of streams, by re- 
ducing the general altitude of a tract of country, brings about 
alterations in the rates of plant movement over such a tract. 
And by the reduction in altitude, changes in annual rainfall, 
annual temperature, mean direction of winds, and in maxima 
or minima of each of thes3, are brought about. Or again secu- 
lar changes in the general level, due to orogenic movements in 
the crust of the earth, may induce greater or less fluctuations 
in the rate of movement of the line of tension, as they are 
