576 The American Naturalist. |. [July, 
loved the heat, the Conifers the cold, and the Hardwood trees 
the happy medium.  Conifers luxuriate to-day in the torrid 
zone, and Hardwood trees and modern congeners of the Palms 
once grew together in Greenland. No innate partiality for 
heat or cold separated the three great groups, but the stern 
laws of plant dynamies that determine the course of the 
struggle for existence. The old established and all-powerful 
tree-group, the patriarchs of the forest, were the Conifers, the 
group best fitted to stem the tide of change and battle with 
opposing conditions; next them in power, because most like 
them in character, were the Diclinae; and weakest were 
the Palms, the group whose foot hold was most precarious. 
These last could hold their own against the powerful Conifers 
and Diclinae only so long as climatie conditions were most 
favorable. Consequently, as the cold advanced from the 
polar regions the palms retreated toward the torrid zone. 
Here they took their stand, their highly specialized structure 
asserted its full power, and gradually they crowded out 
the Conifers and Diclinae, and established preeminent domin- 
ion over the equatorial belt. The Diclinae and Conifers 
were crowded out, *not that they loved heat less, but tliat 
they loved freedom more.” They were fitted to maintain 
themselves against the cold of extratropical regions, and in 
these regions they were relieved from the struggle with a. 
powerful competitor, the whole family of Palms and its 
associated rank luxuriance of tropical vegetation. In short, 
the strength of the Palms when congested into the equatorial 
belt, more than counterbalanced the loss sustained by the 
coniferous and hardwood trees in the cooling of extra- 
equatorial regions. And so the Palms, and with them the 
remnant of their ancient allies, the Tree-ferns and Cycads, 
claimed the tropics for their heritage. There was probably no 
region of the world where Conifers had not gained a strong 
foothold in the long course of ages; there is scarcely a corner 
of the modern plant-world that does not hold some group of 
them ; and it wasthe Conifere that obstinately held their own 
against the cold of sub-polar lands, with the stubborn endur- 
ance that four great eras of geologic time have helped to build. 
4 
3 
P. 
