86 
BOTANY: C. /. CHAMBERLAIN 
mechanical work, but the cycle was incomplete. When erosion super- 
vened and conferred upon the superficies of the continents a certain 
mobility and kinetic energy, the cycle was completed and the stage 
answering the adiabatic expansion was supplied. 
It is true that the efficiency of this engine must be very small, but the 
store of energy upon which it draws — the available boiler capacity — is 
enormous. The mechanism thus appears competent to bring about all 
of the dynamical effects with which geology has to deal. 
J H. Nagaoka, Phil. Mag., 50, 53 (1900). 
2 Simultaneous joints. Froc. Wash. Acad. 7, 267 (1905). 
3 Becker, /. Wash. Acad., 4, 429 (1914). 
^ Ibbetson, Math. Theory of perfectly elastic solids, 1887, p. 174. 
A PHYLOGENETIC STUDY OF CYCADS 
By Charles J. Chamberlain 
DEPARTMENT OF BOTANY. UNIVERSITY OF CHICAGO 
Read before the Academy. December 7, 1914. Received January 1. 1915 
In any phylogenetic study it is desirable to compare related forms 
which are widely separated in time and space. In case of the cycads, 
such comparisons are becoming possible. The general plan of an inves- 
tigation, begun more than ten years ago, is to study numerous forms 
in the field; study life histories in all parts of the family; and then, by 
comparative study, to gain some idea in regard to relationships and also 
in regard to the behavior of various structures during great periods of 
time. 
A typical cycad, with its unbranched trunk and crown of pinnate 
leaves, looks like a tree fern, an aspect which dates back as far as any- 
thing is known about the phylum. 
In the Carboniferous, two great lines of seed plants were abundant 
and widely distributed. One of them is represented today by such 
famihar forms as the pines, junipers, araucarias and taxads. They are 
mostly evergreen, are widely distributed, and form immense forests. 
Being easily available, they have been thoroughly studied, especially 
the north-hemisphere genera. The other line is represented by a single 
living family, the Cycadaceae, or cycads, as they are commonly called. 
The Paleozoic ancestors of the cycads had a world-wide distribution 
and made such a display that botanists have called the Paleozoic the 
Age of Ferns, since the leaves were very naturally mistaken for fern 
leaves. These fern-like seed plants undoubtedly came from an even 
earlier fern ancestry and, in the Carboniferous, existed side by side with 
