1922.] N. Annandale & S. I,. Hora : Parallel Evolution. 509 



run at right angles to it. In all essentials the structure described 

 above is similar to that of the free borders of the disc of Garra. 

 The spine-like outgrowths help to make the surface rough in such 

 a way that better grip must be obtained. 



The minute structure of the disc is thus much less compli- 

 cated than that of the adult Garra or Glyptothorax} The spine- 

 like outgrowths on the organs of adhesion are strictly analogous in 

 the fish and the tadpoles, but they occupy a different position and 

 the structure of the underlying parts is completely different. 



One of the most interesting features of these instances of 

 parallel evolution lies in the fact that whereas in the larvae of 

 the Ranae Formosae we only know, so to speak, the finished product 

 of evolution in the highly perfected organ of adhesion, in the 



epd. / 





c.t. \ 



KIG. 2. — Transverse section through free border of disc of Rana afghana. 

 (Highly magnified and slightly diagrammatic). 

 •s.=spine ; ep. rf.=epidermis ; c. £.=connective tissue. 

 (cf. Rec. hid. Afus. XXIV, p. 50, fig. 13.) 



genus Garra we have before us almost every possible stage alike 

 in postembryonic development, in individual variability and in 

 specific differentiation. One 1 of us has so recently given the 

 facts that it is unnecessary even to recapitulate them here. The 

 evolution of the mental disc of Garra is in this respect parallel 

 to that of oral suckers in various tadpoles of the Himalayan 

 streams. We have thus evidence that these particular structures 

 have come into existence, not through mutation and not by any 

 Mendelian segregation of characters, but through a gradual accu- 

 mulation of small changes. The close correlation, especially in 

 Garra , between these changes and differences in the flow of water in 

 which species and even individuals live is at any rate suggestive. 

 Whether we are witnessing the survival of the fittest in the 

 Darwinian sense or must accept a frankly Lamarckian explanation 

 only experiment can prove. 



' Cf. Hora, op. cit., figs. 12, 13, p. 50; also figs. 15, 17, 18, 19 on pages 53, 

 55. 57- 2 Id., ibid., XX H, pp. 639-643, text-fig. 1 (1921)- 



