INTERPRETATION OF FOSSIL PLANTS 
17 
lack of its observance has been a frequent source of error in the 
past. Reference is made to the lack of precision with respect to 
habitats in making comparisons with the existing relatives of fos¬ 
sil plants, and this applies particularly to the existing plants of the 
equatorial zone. There are many types common in fossil floras, 
such as palms, tree-ferns, gleichenias and matonias, which are 
practically confined to the Equatorial Zone at the present time but 
which are not tropical in the sense that they are confined to the 
tropical altitudinal zone, but which really find their optimum con¬ 
dition for existence in the sub-tropical, or even the temperate, 
altitudinal zone. This is notably true of tree-ferns, and of many 
angiospermous types commonly considered as indicating tropical 
climates in the past. The Aphlebiae of the Paleozoic coal swamps 
are not at all indicative of heat, but of humidity, and the same is 
true of the development of the so-called dripping points of dicotyle¬ 
donous leaves. The cycadophytes which are so prominent an 
element in the flora of the older Mesozoic strata are not indicative 
of either moist or of hot climates, if conclusions may be drawn 
from the existing cyads, but show rather dry climatic conditions 
with abundant insolation. These conditions, rather than low tem¬ 
perature, are what I would infer from the excessive development 
of Ramentum in the Early Cretacic Cycadellas. 
Since the days of Lyell terrestrial plants have been regarded 
as better indices of past climatic conditions than any other class 
of organisms. That they do not always merit this high opinion 
is not because this conclusion is faulty, but because they are so 
often interpreted in a faulty manner. Plant fossils have this 
merit, aside from any question of botanical identification, and this 
fact is usually lost sight of by hostile critics, namely, that the size 
and form of leaves, their texture, the arrangement and character 
of the stomata and other structures afiford criteria that are quite 
as valuable climatically even though the species or genus to which 
they belong remains unknown. 
