210 
In connection with its exhibit illustrating, with living plants, the 
probable course of plant evolution, the Brooklyn Botanic Garden 
has installed, in Conservatory [louse No. 2, the series of eight 
1ese depict some of the 
y 
a 
fac, 
transparencies (shown in Figures 1-8). 1 
myriads of extinet plants that have moved across the world’s stage 
and summarize the main features of the evolution of the plant 
world, 
1. Principal Lines of Organic Evolution * 
As in the beginnings of human history we depend upon tradi- 
tions crystallized into mythology, so in tracing backward the vari- 
ous plant stocks we reach a point where Facts fail us, and we are 
forced to speculate on the earlier stages of plant evolution which 
are forever hidden from us. 
The relations of the principal groups of both plants and animals 
are set forth in the first transparency, which aims to summarize 
what we know and what we surmise. In this great tree of life the 
known is shown in white and that which is surmised is shaded. 
More than half of geological time had passed before there was any 
by that time not only had life 
a 
abundance of fossils in the rocks, anc 
evolved into plants and animals, but nearly all the groups o 
vertebrate animals had already been differentiated, and the ancient 
iwi- 
waters contained a great varietv of algae. 
Students of fossil plants differ widely in their views regarding 
the time when plants first came out of the water and became 
adapted for living on the land. The problem is made difheult by 
the fact that land plants have not been found fossil in any certainly 
recognizable forms until the comparatively late Devonian period, as 
shown on the diagram. ‘This contrast between the early animal and 
plant histories is due to the marine habits of the invertebrate groups 
and to the fact that most of the earlier preserved sediments are o 
marine origin and consequently would fail to preserve any abun- 
dance of land forms even if they were in existence. 
We may, therefore, formulate a brief paragraph on the begin- 
nings of plant life, unhampered by facts, and guided by what seem 
to be the probabilities. Certainly the beginnings of lite were 
not multicellular. We believe that these first or- 
fa 
unicellular anc 
readings, centered on the pages, refer to the full page 
— 
* The numbered 
illustrations of this Guide, 
