1894.] The Meaning of Tree- Life. 577 
The Diclinae retreated before the advancing cold into more 
temperate climes, retreated in fact until they gathered strength 
to wage equal battle with their mighty coniferous opponents. 
Here, in the temperate zones, the Diclinae stood fast and 
crowded the Conifers outward toward the polar regions, not 
toward the equatorial, for there the odds againt the emigrants 
would be tenfold increased. The record of this battle of the 
trees is stamped upon many of the forest monarchs that we 
marvel at to-day. A recent writer has well said: “Just as in 
the formidable armor of some extinct armadillo one may read 
somewhat of its struggles with its enemies, so in the one hun- 
dred meters of solid trunk and in the massive girth of a living 
Sequoia gigantea, the giant red-wood, one may learn of its 
struggles in the ancient forests of Cretaceous and Tertiary 
times, when its allies and competitors were alike more numer- 
ous.” 
The third great tension system is now unfolded before us. 
We see the hardwood forests of temperate regions facing on 
the one hand the congested luxuriance of equatorial vegeta- 
tion, and on the other the ancient coniferous forest gathered 
round the poles and step by step forced backward by advanc- 
ing cold. There is a great equatorial pressure toward the 
poles, and an opposing polar pressure, traceable to opposite 
causes; and between them there is a broad tension line, the 
temperate zones. Conway MacMillan, who was quoted just 
above, has proposed a broadly generalized division of the 
world into two great botanical realms, the Central Realm and 
the Distal Realm. But the division should be carried a step 
farther; taking the three great forest elements as a guide, we 
may fully express the evolutionary history of plant dynamics 
by recognizing three great divisions :— 
The Central Tropical Realm, the Tensional Temperate 
Realm, the Distal Sub-Polar Realm. The three merge into 
each other and their elements are everywhere somewhat com- 
mingled, but in the main they are fairly distinct. Such was 
the general plan of the plant world of the late Tertiary, 
proximate Preglacial times. The Glacial Period had a 
wonderfully interesting effect in So the northern 
