466 The American Naturalist. [June, 
the orchids, among Monocotyls, and the kinsmen of the golden- 
rods and asters, among Dicotyls ; as the monarchs of vegetative 
energy stand the tree-ferns and towering palms of the tropics, 
the Red-Woods of California, the Eucalyptus of Australia, and 
our forests of mighty oak. Thus the classification systems of 
modern botany that review in the clearest scientific light the 
evolutionary relationships of species, genera, orders, and classes, 
present to us only one side of the problem of plant-life; the 
dynamics of vegetation is the other. We may know that the 
Coniferae are among the most primitive of flowering plants, 
and the Orchidacese and Composite among the highest; 
but why do we find our orchids and composites growing as 
stunted herbs in the very shadow of conifers that are giant 
trees? Which is master of the situation? Systematic botany 
has not fulfilled its mission until it has grasped both sides 
of the two-fold relation that the contrasting types bear to one 
another. ; 
Far back in geologic time the dawn of tree-life came almost 
with the beginnings of vegetation. What the earliest of those 
beginnings were we can scarcely even conjecture, but going 
back as far as fossil botany will carry us with certainty, we 
may conceive something of the conditions under which the 
primitive plant-world was fostered. Consider the probable con- 
ditions of the Cambrian and Silurian Ages. However, scant 
the records that we hold, they are yet sufficient to give us some 
suggestions of inestimable importance. Past question the 
earliest forms of plant-life were denizens of the water, developed 
in the seas and lakes of Pre-Cambrian times. From their 
aquatic habitats they must have first gradually emerged, a8 
the cooling of the primordial continents permitted, and the 
strengthening of their own anatomical characters favored, 
into the swamps and marshes, and then step by step mounted 
the higher regions of the dry land. The oldest fossil types We 
with certainty know of were far from the beginning of the 
scale; they could only have been products of ages of develop- 
ment that must forever remain to us almost a total blank. 
The world of Silurian times was probably a torrid zone from 
pole to pole, a condition traceable in large part to the insular 
