2 10 
Douglas Houghton Campbell. 
their present distribution, must be relatively old ones, have left few 
or no fossil traces. The Violaceae and Begoniaceac may be men¬ 
tioned as examples of such families. The genus Begonia is wide¬ 
spread throughout the tropics, and extends little, if at ail, beyond 
them. Absolutely no fossil remains of either the extremely 
characteristic leaves or fruits are known. As these plants occur where 
one would expect that they might be preserved fossil, the only 
explanation to be offered of their absence in a fossil state is that 
their tissues are too delicate and perishable to have left any traces. 
(See Warburg, Begoniaceae, Engler and Prantl, III. Th. 6a Abt., p. 
133). As no Liverworts approach in robustness most species of 
Begonia, it is quite as reasonable also to explain their great scarcity 
in a fossil condition as the result of their extremely perishable 
nature. 
As a rule, the Hcpaticae are not especially adaptable plants. 
Growing usually in moist shady places, they are not fitted to traverse 
extended dry or exposed areas. It is commonly assumed that their 
light spores are fitted for rapid dispersal by wind, but in many of 
them—and these including some of the most widely distributed 
genera—the spores have very delicate thin walls and contain chloro¬ 
phyll, showing that they are adapted only to immediate germination ; 
such spores very soon lose their power of germination. Unfor¬ 
tunately very few data are available on this point, but some experi¬ 
ments made by Mr. H. B. Humphrey, under the writer’s direction, 
showed that none of the spores of this type that were examined 
retained their vitality more than a few weeks, and in the widespread 
Fegatella conica a very few days’ drying quite killed all of the spores. 
It is highly probable that further experiments on the germination 
of the spores of most species growing in the wet tropical forests 
will show that the spores very soon lose their power of germination, 
and will not survive any prolonged dessication. 
It is true that such xerophilous species as Targionia Jiypophylla 
and Fimbriaria Californica have spores fitted to survive long periods 
of drought, but these are, with little question, secondary forms 
adapted to a special environment. Some of these xerophytic forms 
show other modifications due to their peculiar environment. The 
characteristic tubers of Geothallus and of some species of Fossom- 
bronia, are adaptations to extreme xerophytic conditions. 
We can hardly explain the present distribution of such wide¬ 
spread tropical genera as Dendroceros, Monoclea, and Dumortiera 
by the theory that their spores could be carried across the wide 
