SummaryA comparative study of the Phaeophyceae and of the Chaetophorales (Chlorophyceae), which exhibit a development parallel to that of the simpler members of the former group, shows that, both as regards increasing vegetative specialization and elaboration of the life cycle, definite trends can be recognized in the evolution of the larger multicellular Brown Algae. The heterotrichous state, which is manifested in its simplest forms in the Chaetophorales and in the Ectocarpaceae, is met with in the juvenile stages of all the numerous remaining Ectocarpales. In these the erect filaments either become aggregated in various ways to form pseudo‐parenchymatous thalli, in part of some size, or undergo septation in diverse planes to give simple parenchymatous forms. Anatomical specialization, for example in such a series asPhloeospora‐Stictyosiphon‐Scytosiphon,arises among these parenchymatous forms by the restriction of cell division to the peripheral layer of the thallus and the marked elongation of the inner cells. The sporophytes ofChorda,formerly included in Ectocarpales, differ from those of the more specialized parenchymatous members of the latter essentially only in the disappearance of a prostrate system in the juvenile stages and in the presence of ‘trumpet‐hyphae’.Chordathus constitutes a connecting link between them and the series of large and elaborate forms characterized by marked surface development comprised in the remaining Laminariales. It is postulated that the ancestors of the Laminariales were simpleStictyosiphon‐liketypes which developed in one direction intoChordaand in another into a form likeLaminaria saccharinafrom which all other Laminariales can be derived.The Fucales represent a different and more highly specialized evolutionary line in which external elaboration has taken place by branching, whereas in the Laminariales it results from splitting of a primarily entire thallus. The anatomical resemblances between the two orders are possibly due to evolution from a commonStictyosiphon‐likeancestor. There is some evidence that the apical growth of Fucales may have been secondarily derived from the intercalary growth usual among Ectocarpales. In the possession of tetrahedral apical cells, whose segments contribute largely to the building up of the younger parts, and in the differentiation of compact tissues, with little mucilage‐formation and with hyphae developing only in the oldest parts, the bulk of the Fucales (Cystoseiraceae, Sargassaceae) show an anatomical organization approximating more to that of simple archegoniate plants than to that of the Laminariales. The Fucaceae, usually regarded as typical of the whole order, diverge in their four‐sided apical cells, their prevalent dichoto‐mous branching, and the copious mucilage‐development, the last feature being possibly related to the littoral habitat.The change from a haploid to an isomorphic life cycle, previously established in various Chlorophyceae, is now known to occur also in the heterotrichous Chaetophorales. A similar change no doubt occurred among Phaeophyceae, but no living haploid forms are known in this class. The ancestral haploid types probably bore only plurilocular sporangia producing either zoospores or gametes. When an isomorphic phase‐alternation became established, these two structures will have persisted on the respective phases, while on the diploid plant a new organ, the unilocular sporangium which became the seat of meiosis, was differentiated, perhaps arising by modification of a plurilocular sporangium. Plurilocular and unilocular sporangia remain associated on the diploid phases of most Ectocarpales and some Sphacelariales, but in other Brown Algae the former have disappeared.While the comparatively simple Ectocarpaceae have an isomorphic life cycle, the majority of the Ectocarpales have a heteromorphic one, in which the sporophytes have alone been elaborated, while the gametophytes appear as diminutive filamentous growths. The same condition is met with in Desmarestiales and Laminariales where, however, accompanying the greater vegetative specialization the isogamy of Ectocarpales is replaced by an oogamous sexual process. The Sphacelariales, whose simpler members are related to the Ectocarpaceae and in which both phases have undergone equal elaboration, show a similar progression from isogamy to oogamy in the more highly differentiated types. It is thus possible to trace an advance from Ectocarpales to Laminariales in respect of(a)increasing vegetative specialization including suppression of the prostrate system in early stages,(b)adoption of a heteromorphic life cycle, and(c)evolution of an oogamous sexual process.The Fucales stand apart from other Brown Algae, not only in vegetative organization, but also in the occurrence of sex organs on the diploid phase, their development within conceptacles, the presence of more than one ovum in the oogonium, and the elimination of gametophytes. The Fucales appear in nearly every respect to be more advanced than other Phaeophyceae and, although they no doubt originated fromEctocarpus‐likttypes, it is more difficult to trace their mode of origin than in the case of the Laminariales. There is some reason to believe that their distinctive life cycle may have originated from the tendency of the normally asexual swarmers of the unilocular sporangia to behave as gametes which has been established in a number of the less specialized Brown Algae.The progressive elaboration, which can thus be traced among Brown Algae, probably shows considerable parallelism with the changes which occurred during the evolution of archegoniate plants, although the former exhibit many peculiarities of their own (e.g. the meristematic surface layer, hyphae) which appear already