334 THE POPULAR SCIENCE MONTHLY 
present the least variable features in those factors which determine re- 
lations to mechanical stress and physiological needs. 
External organs are notoriously subject to variation, even under 
slight alterations of surrounding conditions, within the limits of the 
Species or even within various stages of development of the same indi- 
vidual. From this it is clear that organs such as leaves must be very 
unreliable for phylogenetic purposes. It is, unfortunately, true that 
much of the paleobotanical work based upon a study of such parts must 
be of inferior value, and the conclusions drawn will require extensive 
revision when the more rigid tests to be applied through a knowledge 
of the stem structure are brought to bear. 
The value of paleobotanical evidence consists in its ultimate corre- 
lation with known types of plants, and it is obvious that all such studies 
should be prosecuted with direct reference to the broader require- 
ments of plant biology. This involves a comprehensive knowledge of 
the history of plant life from its earliest development; that the data 
derived from a study of living species should be correlated with the evi- 
dence obtained from fossilized remains. Existing vegetation shows a 
very incomplete record of plant life as a whole. Its history as known 
until very recent times, and even now to a very large extent, is dis- 
played only through the medium of detached groups, and relates chiefly 
to the most highly organized types. Through the perspective afforded 
by paleobotany, it becomes possible to not only supply missing facts, 
but to establish what theory has for so long a time required a satisfac- 
tory demonstration of—a more or less continuous series of phenomena 
from the rudimentary forms to the most advanced organisms. 
Until a very recent date the Linnean division of plant life into two 
great phyla, the cryptogams and the phanerogams, was the prevailing 
conception of the constitution of the plant kingdom. This division 
recognized no connection between the two great groups, but regarded 
them as wholly distinct in origin as in character. But the rapid ad- 
vances in a knowledge of plant anatomy, developed toward the middle 
of the last century, and especially the remarkable and epoch-making 
observations of Hofmeister respecting the process of reproduction, 
enabled him to break down the old barriers erected by the doctrine of 
the constancy of species, and prove a genetic connection between the 
primary divisions of Linneus. With this starting-point, the crypto- 
gams and the phanerogams were subjected to a severe scrutiny from an 
entirely new point of view, with the result that each underwent a re- 
vision which led to such a rearrangement of subdivisions as to present 
an entirely fresh conception of their relations to one another. The 
logical result was finally expressed in the subdivision of the plant world 
into four great phyla, which, in their evolutional sequence, came to be 
known as I., Thallophyta; II., Bryophyta; III., Pteridophyta; IV., 
Spermatophyta. 
