38 
IRVING W. BAILEY AND EDMUND W. SINNOTT 
of the total ancient vegetation of which it once formed a part. 
Furthermore, the entire-leaved portion of a flora may contain a greater 
or less number of large- and small-leaved types, depending upon the 
exact nature of the climatic and edaphic influences that may have 
been operative. Therefore, in comparing fossil with living floras, it is 
important to take into consideration the number of megaphyllous 
and of microphyllous types that are represented. It should be noted, 
however, that this method of studying the climates of the Cretaceous 
and Tertiary rests upon a physiological and ecological basis rather 
than upon the usual phylogenetic one. It promises to afford a simple 
and rapid means of gauging the general climatic conditions of the 
Cretaceous and Tertiary, and of checking the accuracy of conclusions 
derived from other lines of evidence. 
In conclusion, it may well be asked, what bearings this study of 
the distribution of entire and non-entire leaves and leaflets has upon 
the question of the relative conservatism of the leaf? As far as the 
leaf-margin of Dicotyledons is concerned, it may be very variable or 
extremely inconstant. For example, so long as entire-leaved plants 
remain in habitats that strongly favor the formation of entire margins, 
foliar form will remain unaltered, and the leaf will appear to be quite 
conservative. The most variable conditions, on the other hand, seem 
to occur in intermediate environments, or among entire leaved plants 
in non-entire environments, or vice versa. 
It cannot be too strongly emphasized that there is grave danger 
in inferring that, because a certain character has remained unaltered 
in one or more groups of plants through long periods of geological 
time or has varied greatly among certain closely related forms, the 
organ which possesses it is inherently "conservative" or "inconstant." 
In any organ, not all characters are necessarily equally stable or vari- 
able. Furthermore, the same character may vary in its constancy 
or instability in different environments, in different plants, and at 
different stages in the ontogeny of the individual or in the phylogeny 
of the genus or family. Although characters of little or no apparent 
functional importance have been shown in certain cases to be more 
conservative than others, that are subject to the influences of environ- 
ment, it should be kept in mind that a functionally important char- 
acter may long remain unaltered, if it is subjected to an unchanging 
environment. Continuity of similar environmental influences is 
responsible, in all probability, for some of the most striking cases of 
