1032 ELLIOT ROWLAND DOWNING 
this is true, too, in the algae, as already pointed out. Let us 
review the facts down into that group with diagrams. 
The mosses, (diagram ITT), show a sporophyte and gametophyte 
generation of about equal importance. The egg and sperm are 
usually borne on the same gametophyte. 
The sporophyte becomes less and less prominent in the arche- 
goniates as we approach the algae. Among the green algae the 
intercalation of a sexual generation seems the rule rather than the 
alternation of generations. The life history of Spirogyra, of the 
Diatomaceae, and of such protozoa as Chlamydomonas and Poly- 
neratio 
on 3 1s of o 
@ °o 
oe ~) 
° 
J 
Zygote 
ce} 
2 
y °o 
“VNeguctfon S 
X 
O Gametes x 
\ \ 
yds a 
‘ 
\ °. 
26, 
\ 
wn 
s\ =, 
ox SI 
%>, > 
ON 
Fig. 7. Diagram V. Scheme of the life history of Adelia, Monas, etc. 
toma would be graphically shown in diagram IV. Insuchformsas 
Adelea, Monas and Actinophrys among the protozoa, in no algae 
so far as I know, reduction occurs during the fusion of the gametes; 
the life history is given in diagram V (text fig. 7). 
Nowwhich of these life histories most nearly approximates prim- 
itive conditions? The question involves a discussion of the rela- 
tion in phylogeny of the alternation of generations, sexuality and 
reduction. The evidence already given shows that the latter 
two phenomena antedate the alternation of generations, for both 
are found among simple animals and plants that have not-achieved 
the alternation. 
