XXXVII] COMPARISON 37 



linkage, and completes it physiologically. There is reason to believe from 

 its sporadic occurrence, even in individual leaves, that linkage has been 

 initiated repeatedly and independently in individuals, species, and genera 

 of Davallioid Ferns. The facts indicate this for Diellia, Tapeiniditmi,Nephro- 

 /epis, and Lindsaya. If this is so within the Davallieae, there will be no need 

 to see in such linkage any proof of their relationship with the Pterideae, 

 though it is in them that linkage finds its highest development as seen in 

 Ferns of marginal origin. 



These three modifications of the sorusgo along with various other structural 

 details already noted as leading to a full Leptosporangiate state. Moreover, 

 they are all physiologically advantageous. The transit from a marginal to a 

 superficial position of the sorus gives protection, especially perhaps from the 

 incidence of intense light. The abortion of the lower indusium may be held 

 as the economic removal of a part that has become superfluous in consequence 

 of that transit. The linkage, with its vascular commissures, gives the advan- 

 tage of equalising supplies available for any individual leaf or segment. In 

 addition to these changes there is the passage from the gradate to the mixed 

 type of sorus. This secures the spread of the drain of spore-production over 

 a longer time and a larger space. It goes along with the elongation of the 

 sporangial stalk. It is true that this carries the mature sporangium farther 

 from the source of supply: but as it approaches maturity it has little further 

 need of food, while by the elongation it secures the shedding of the spores 

 into the free space above the younger and shorter-stalked sporangia. Thus the 

 structural advances which mark the Davallioid Ferns may be held not merely 

 as morphological features that serve for classification, but as evolutionary 

 advances that have come into existence as sources of physiological advantage 

 They possess in fact "survival value." 



With these general considerations as a basis the Davallioid Ferns may be 

 grouped in relation to the Dennstaedtiinae, which form an outlier of phyletic 

 advance towards them from the Dicksonioid source. Three natural sections 

 may be distinguished by their soral condition. 



' I. Primitive individuality of the sorus retained (except in some 

 species of Nephrolepis): position more or less marginal. Dermal 

 scales. 



(i) Htiinata {CdiV., 1802) ... ... ... 14 species. 



Leaves once or more pinnate, venation free. Sori intra- 

 marginal, edges of inner indusium free. 



(ii) Davallia (Smith, 1793) (reduced) ... Gj species. 



Leaves variously pinnate, venation free. Inner indusium 

 with edges fused to leaf-surface. 



