98 THE HISTORY OF CREATION. 
thallus, also called Protonema, still remains in that lower 
stage of elaboration manifested throughout life by all Thallus 
plants ; that is to say, stem and leaf-organs have as yet not 
differentiated, and the entire cell-mass of the Fore-growth 
corresponds to a simple thallus. The second and more 
perfect generation of mosses and ferns—the Stem, or Cormus 
—develops a much more highly elaborate body, which has 
differentiated into stalk and leaf (as in the case of flowering 
plants), except in the lowest mosses, where this generation 
also remains in the lower stage of the thallus. 
With the exception of these latter forms the first generation 
of Mosses and Ferns (the thallus-shaped Fore-growth) always 
produces a second generation with stem and leaves; the 
latter in its turn produces the thallus of the first generation, 
and so on. Thus, in this case, as in the ordinary cases of 
alternation of generation in animals, the first generation is 
like the third, fifth, ete, the second like the fourth, sixth, 
ete. (Compare vol. i. p. 206.) 
Of the two main classes of Prothallus plants, the Mosses 
in general are at a much lower stage of development than 
the Ferns, and their lowest forms (especially in an anatomical 
respect) form the transition from the Thallus plants through 
the Algze to Ferns. The genealogical connection of Mosses 
and Ferns which is indicated by this fact can, however, be 
inferred only from the case of the most imperfect forms of 
the two classes; for the more perfect and higher groups of 
mosses and ferns do not stand in any close relation to one 
another, and develop in completely opposite directions. In 
any case Mosses have arisen directly out of Thallus plants, 
and probably out of Green Algze. 
Ferns, on the other hand, are probably derived from 
