The Distribution of the Hepaticce. 
207 
Asa Gray, edited by C. S. Sargent, 1889) on the correspondence 
between the floras of Pacific Asia and Atlantic North America, 
show the survival in these widely separated regions of many wide¬ 
spread Tertiary genera that have quite disappeared from Europe 
and from Western America, where there is positive evidence that 
they once abounded. While the fossil evidence in this case is 
confined to woody trees and shrubs like Liriodendron, Magnolia 
and Hamamelis, it is reasonably certain that other peculiar genera, 
like Podophyllum, Mitchella, Diphylleia, and Nelumbium, are of 
equal antiquity, and were probably as widely spread as the trees 
* 
and shrubs associated with them at the present day. The absence 
of these more delicate plants in a fossil condition must he home in 
mind in dealing with the question of the Hepaticae, which are very 
much more delicate and perishable than any of these herbaceous 
angiosperms, and consequently even less likely to leave fossil 
traces. 
The diversity of species grows more marked in the tropical 
regions, and the number of genera and species common to the 
tropics of the Old and New Worlds is relatively small. The wide¬ 
spread genera, like Euphorbia, Ipomoea, Solanum, Polypodium, 
are mostly extratropical as well. The number of strictly tropical 
genera, common to both hemispheres, is very small. 
The very wide-spread genera, e.g., Ranunculus, Ficus, Senecio, 
are either members of families generally recognized as primitive 
ones, or else, as in the case of Senecio, are exceptionally adapted 
to rapid dispersal. It is undoubtedly the latter trait that explains 
the extraordinary predominance of Composite in the modern flora. 
We may safely conclude that many existing families and genera 
of Angiosperms were already differentiated in Tertiary times and 
have come down to the present with little change. Many genera, 
however, and probably most existing species, are of more recent 
origin, and the changes have naturally been most rapid in the 
warmer parts of the world. The present distribution of most of the 
highly specialized types, like most Orchidaceae, Zingiberaceae, 
Bromeliaceae, etc., indicates that these are of relatively late origin, 
and are not closely allied to the more primitive types of Angio¬ 
sperms. 
The Hepaticae, including the Anthocerotaceae, comprise, 
according to Schiffner (Engler and Prantl, Nat. Pflanzenfamilien 
I Th., 3 Abt., p. 141) not quite 4,000 species, Since the date of 
this enumeration (1895) the number has been considerably 
