Davis.-— The Origin of the Archegoninm. 483 
organs unlike any existing Thallophytes ? The justification 
could only be the theoretical working-out of a very plausible 
series of stages in types whose previous existence, while en- 
tirely speculative, would do no violence to the position and 
arrangement of existing groups of Algae. 
The assemblage of plants called the Thallophytes is much 
better understood with the advances of recent years. Although 
correlative with three other branches of the plant kingdom 
(Bryophytes, Pteridophytes, and Spermatophytes), the Thallo- 
phytes are peculiar in quite lacking common morphological 
characters of the sort that make these assemblages of higher 
plants very natural groups. The bonds of union in the 
Thallophytes are negative characters. Its members do not 
have the various positive marks of the higher groups. The 
association of the Thallophytes together because the vegetative 
structure is generally undifferentiated into stem, root, and leaf, 
is very similar to that old grouping of several independent 
branches of the animal kingdom under the head Invertebrata 
because they lacked the character of the highest subdivision. 
The Thallophytes include an immensely more diverse 
assemblage of subclasses and orders than any other great 
class of plants. These groups are in certain regions so 
distantly related to one another that the gaps can only be 
bridged by assuming the previous existence of whole orders 
now entirely extinct or represented only by an occasional 
stray remnant. And the ages that brought about this frag- 
mentary condition, with its remarkable forward developments 
in various directions, have left us in the structure of the 
surviving forms little or no evidence of the exact steps in the 
process. It is necessary to state this standpoint with respect 
to the Thallophytes so that the reader will clearly understand 
the possibilities of the theory that will be discussed presently, 
and which perhaps demands such preliminary explanation to 
justify its speculations. 
The Chlorophyceae, as we have said, present no multi- 
cellular organs from which the archegonium or antheridium 
can be easily derived. But one region of the Thallophytes 
