1894. ] The Evolution of the Hepatice. 353 
may have been strongly influenced and perhaps rapidly 
evolved by the peculiar environment of precarboniferous 
times; at least the statements of the books in reference to the 
excessive amount of carbonic oxide in the atmosphere being 
this point, however, it may be questioned whether the state- 
ments of the books do not need some modification. _ 
I have said that the Hepatice have undergone a triple dif- 
ferentiation. Commencing with a simple thallose plant with 
its unmodified sporogone, it is evident that there are three 
possible lines of specialization: (1) the development of the 
thallus as such; (2) the transformation of the thallus into a 
leafy axis combined with the modification from creeping to 
ascending or erect habit; and (3) the specialization of the 
Sporogone at the expense of the thallus. Even a cursory ac- 
quaintance with the diverse structures that are developed in 
the group will make it evident that the Hepatice have im- 
proved their opportunity in each of these three possible lines 
and have carried the differentiation of each line to a high de- 
gtee of perfection. Let us follow out in some detail these 
three lines of development. 
The Marchantiales. We must place as lowest in the 
series the group which commences with such simple types as 
Riccia and Tesselina and ends with the elaborate Marchantia 
and its congeners. Among the lowest types the habit is not 
greatly different from that of the alge, the plants either float- 
ing in water or attaching themselves to wet soil. The cap- 
sular development in the lower forms moreover is not very 
diverse from that of certain of the higher alge, the sporogone 
ing without stem and often imperfectly surrounded by a 
Capsular wall. As we advance to higher forms, we find not 
only an extensive modification of the thalloid structure neces- 
Sitating an elaborate system of stomata and in many Cases 
‘pecially modified branches for the better accommodation of 
the reproductive bodies, but also a striking advance in the 
Sapsular development in which the egg cell develops not only 
* capsule or fertile portion, but also a stalk or sterile portion, 
which with the addition of elaters formed within the capsule, 
*tter serves to distribute the spores. eo is 
© may note here also two types of differentiation in the 
‘Pecialized branch that bears the carpocephalum, as it €x- 
