572 The American Naturalist. [July, 
THE MEANING OF TREE-LIFE. 
By Henry L. CLARKE! 
(Continued from Volume 28, page 472). 
It is a striking fact that the older fossil forest remains, at 
least through the Paleozoic and early Mesozoic strata, present a 
wonderful likeness in character the whole world over. The 
wide scattering and spreading of types that this indicates, is to 
be directly accounted for partly by the more frequent physi- 
cal changes that took place in early geologic times, and the 
constant changes and shiftings in the relative positions of con- 
tinental surfaces, through upheavals and subsidences; and 
in part by the wide wind-dispersion possible for the spores of 
the Paleozoic Cryptogams. Past question geology makes 
countless blunders in assigning strata in different parts of the 
world to the same age because of likeness in their fossil flora 
(and the statement holds almost equally true of fauna), 
where likeness is in fact a positive proof that the strata 
are not synchronous. But the chances for error in this 
direction decrease from the latest to the most remote ages. 
All evidences indicate more and more homogeneous climatic 
and physiographic conditions as we trace the geologic record 
farther and farther back. 
When the low insular character of the early continents, and 
the consequent increased humidity of the atmosphere extend- 
ed a nearly sub-tropical climate to the poles, it is obvious 
that the potency of the sun as a maker of the seasons and 
and zones, counted for far less than now,—unless indeed the 
sun itself were tremendously hotter then than now. But that 
this last supposition is false within the history of vegetation 
is proven by asimple fact. Were it true, the equatorial zone 
would have been a region of such intense heat that it would 
have formed an impassable barrier between the floras of the 
1University of Chicago. 
