Flagellata and Primitive Alga. 
groups ( e.g ., Crystoflagellata, Silicoflagellata, Coccolithophoridae) 
not classed by Senn among the Flagellata, though they occupy 
this position in zoological systems of classification. On the hand, 
the Trypanosomes and the majority of the other specialised hete- 
trotrophic Flagellata need not be considered in a discussion of the 
origin of Algae from Flagellata. 
In the following pages, it is proposed, taking as a starting- 
point the treatment of Algal groups in Engler and Prantl’s Pflan- 
zenfamilien and that of the Flagellata in the same work and in the 
more comprehensive zoological treatises, to review briefly the 
advances that have recently been made in our knowledge of certain 
lines of descent leading from Flagellata through transitional forms 
to the simpler Algae. Restricting our speculations to such series 
as appear to include what may be fairly considered as transitional 
forms, we can recognise three main lines, leading (i) from the 
simpler Chloromonads to the Heterokontae or Yellow-green Algse ; 
(ii) from the Polyblepharids to the Chlamydomonads and thence 
to the Isokontae and other Green Algae ? and (iii) from the Chryso- 
monads and Cryptomonads to the Brown Algae, the Peridiniales, 
and probably certain other groups. 
Ill— Chloromonads and Heterokontae: (“Confervales,” 
“Yellow-Green Alg.e: ”). 
Until recently the Green Algae have usually been divided into 
Conjugatae, Protococcoideae, Confervoideae, and Siphoneae—the 
division made, for instance, by Wille in Engler and Prantl’s 
Pflanzenfamilien in 1890, and adhered to, with slight modifications, 
in that writer’s recent supplement on the Chlorophyceae in the same 
work (150). The Conjugatae form a natural group, marked by the 
absence of ciliated reproductive cells—whence the name Akontae 
given to the group in the modern system of Green Algae based in 
part upon the ciliation of the asexual reproductive cells or zoogo- 
nidia—and by the siphonogamic sexual process of conjugation by 
means of a tube formed by the fusion of processes from the two 
gametangia. The limits and arrangement of the three remaining 
classes have, however, been considerably modified, owing chiefly to 
considerations advanced in publications by Chodat, Bohlin, Luther, 
and Blackman. 
In all Green Algae excepting the Conjugatae (Akontae), the 
zoogonidia and zoogametes are typically pear-shaped cells bearing 
at the anterior end a number of flagella. In most cases there are 
two (occasionally four) flagella of equal length inserted at the same 
point (Isokontae); in the small order GEldogoniales the motile cells 
have a circlet of numerous flagella (Stephanokontae) ; while in the 
Confervales there are two flagella of unequal length (Heterokontae) — 
in some forms the shorter flagellum may, apparently, be absent. 
It may be added that in the attempts that have been made to 
connect Algal series with corresponding Flagellate series, the 
number and insertion of the flagella are not the sole criteria used, 
various other characters being taken into account. In certain 
groups of Flagellata, e.g., the Euglenineae, the flagella show con¬ 
siderable differences in genera which are obviously related closely 
to each other as judged by other cytological characters. When due 
attention is paid to the tout ensemble of characters, however, there 
can be little doubt that the flagellum characters (number, insertion, 
