Algae and the Principles of their Evolution. 103 
the three important tendencies which rule over the lower 
Green Algae. 
A. The zoospore-condition , namely, the unicellular motile 
stage, becomes the leading principle in the Volvocineae, in 
which the two other conditions are only transient or sub- 
ordinate. 
B. The sporangium-condition, that is to say the unicellular 
motionless stage with a consistent cell-wall, is in the second 
group the leading character, the two others being only acci- 
dentally realized, or merely transient (Pleurococcoideae). 
C. The Tetraspora-stage , namely, where the non-motile 
cells are connected by regular cell-walls at right angles, or 
nearly so, arising from the division mentioned above, becomes 
a preponderant character in the Ulvaceae and the filamentous 
Algae by the increasing of the consistency of the walls, 
which is the condition of a more definite form of tissue or 
filaments ; whilst the two others are merely transient or 
reduced. 
These principal lines being established, each of them may 
follow a very logical evolution and conform to the leading 
principle. In their lower types there will be a less strong 
fixity of the main characters, and a much greater resemblance 
to the different conditions of the Palmellaceae. 
Volvocineae. The general cell-structure of the Volvocineae 
is the same as in the Palmellaceae, but with variations due to 
their special evolution and to their mode of life. In some 
species of Tetraspora , and especially in the curious genus 
Apiocystis 1 , each cell of the thallus is already provided with 
two cilia, but these cilia are motionless. 
The unicellular Chlamydomonas species, with their gelatinous 
envelope, are propagated by bipartition or multipartition 
within the mother-cell-wall ; the division is at. first longi- 
tudinal, but in most cases very soon becomes transverse by 
the rotation of the separating wall. Very often the four 
daughter-cells show a tendency to take the form of the 
1 See S. Moore, Apiocystis a Volvocinea, Trans. Linn. Soc., Vol. xxv, 1890, p. 362. 
Thuret et Bornet, Notes Algologiques. 
