Thalassiophyta and Algal Ancestry of Higher Plants 171 
plant-life. Why the plant, and not the animal, kingdom has adopted 
this scheme remains unanswerable (cf. also Fucales). 
Among the Seaweeds we meet with great diversity in the character 
of the alternation, a fact which is best exemplified in the Phaeo- 
phyceae (cf. Dictyota and Cutleria for instance). When Church 
remarks (p. 25) “In all cases of heterothallic differentiation, the 
individual derived directly from the zygote is the one which begins 
to diverge from the type, and expresses deterioration,” he overlooks 
Sauvageau’s important work on alternation of the Laminariales 1 . 
Moreover neither the Ectocarpales nor the simple filamentous Green 
Algae of the sea show this phenomenon. Even among the Red 
Algae the Nemalionales exhibit a simple type of “antithetic” 
alternation which is not much removed from that of Coleochaete 
among Green forms, except perhaps in the incidence of the reduction- 
division. The fact that there may be no alternation in a considerable 
bulk of Seaweeds and that there is great diversity in the character 
of such alternation appears to lend support to the view that alter¬ 
nation need not necessarily only have evolved under marine con¬ 
ditions, but has been a phase of evolution of the Vegetable 
Kingdom. 
Although Coleochaete is the only green form that exhibits any 
definite type of alternation, the potentialities for such an alternation 
are, as will be shown below, quite evidently present 2 . The absence 
of definite alternation in most Green Algae (of sea and freshwater 
alike) may be taken to imply either (a) that alternation in green 
plants was evolved subsequent to the adoption of a land-life, or 
(b) that those Green Algae that had already acquired alternation 
became the successful transmigrants 3 . 
In Church’s memoir, where the primeval ocean is supposed to 
have existed before there was any dry land, the origin of a terrestrial 
flora is postulated as occurring in connection with the gradual 
elevation of vast continental stretches of sea-bottom above the 
water-level. What evidence have we that when such elevation took 
place the Benthos had attained to anything like the stage of develop¬ 
ment that modern Seaweeds exhibit? Church has sketched for us 
the fascinating picture of the evolution of Benthic from Planktonic 
forms as the ocean-floor was raised and the sea became locally 
1 Cf. for instance, Comptes Rendus de VAcad. d. Sci. t. 161 , 1915. 
2 Cf. also Fritsch, “The algal ancestry of the higher plants,” New Phytol. 
15 , 1916, p. 233 et seq. 
3 It is however not easy to suppose that in such a case no form exhibiting 
alternation should have been left behind. 
