56 MR. A. W. BENNETT ON THE 
prevented its full success there. Where cell-division originated 
in the Chlorophyllophyces is not clear; we find it already fully 
developed in the Nematophyces or Confervoidex isogame. The 
organisms included under this class consist of a single un- 
branched or branching filament of cells; the only known modes 
of reproduction being, in most cases, the conjugation of zooga- 
metes and the direct germination of larger zoospores. In the 
two lowest families, the Chroolepidee and the Ulotrichacee, 
embracing a very small number of genera (Chroolepus, Trente- 
pohlia, Ulothrix, and a few others), the filament is usually un- 
branched ; in the two higher, the Confervacee, including Conferva, 
Cladophora, Chetomorpha, Draparnaldia, and other genera, and 
the Pithophoracee, made up of the single genus Pithophora, 
further vegetative activity is displayed by the copious branching ; 
and in the Confervacex we have again, in some instances, a 
plurality of nuclei. The exact course of evolution from the 
isogamous Confervoide® is obscure ; but it would appear to have 
taken place in three different lines. The first of these, which 
evidently came to an abrupt conclusion, is the Conjugate, con- 
sisting of the Zygnemacee, Mesocarpea, and Desmidiee, a well- 
marked and sharply differentiated group with no near affinities. 
It seems to me most probable that the first two families, consist- 
ing entirely of unbranched filamentous forms, are derived from 
the Confervoidex directly, though the change in the mode of 
reproduction is very abrupt. The formation of zoospores is 
entirely suppressed, the only mode of reproduction being lateral 
conjugation between cells of the same, or scalariform conjugation 
between those of different individuals. The Desmidiee must, 
then, be regarded as a family adapted, by a certain amount of 
retrogression in both vegetative and reproductive characters, to 
life in shallow water. If this view be correct, they must be 
derived, through such filamentous genera as Desmidium and 
Hyalotheea, from Zygnemacex with lateral conjugation. The 
possible relation of the Diatoms to the Desmids I have already 
discussed. I believe it to be apparent rather than real The 
mode of reproduction by conjugation attains its climax in the 
Mesoecarpes, which are distinguished from the Zygnemaces 
mainly by the more complex mode in which the zygosperm is 
formed. 
The second line of descent is that of the Brown Seaweeds. 
The formation of some coloured pigment to mask the chlorophyll 
