Variation of the Edible Frog. 251 



extending to the hip (fig. 5, ;i), others have a detached posterior 

 part as in the typical form (c), whilst others again (b) con- 

 nect the two conditions, the posterior part of the fold, though 

 deviating, being confluent with the anterior and forming a 

 bend before reaching the thigh. 



Bollavv mention* among the specific differences between 

 R. esculent a, R. ridibunda, and R. chinensis, that the dorso- 

 lateral fold is wider (as wide as the upper eyelid) in the 

 second than in the two others. This character is absolutely 

 worthless, for in specimens of the typical form from France 

 and Switzerland its width usually measures i to § that of 

 the upper eyelid, but may be equal to it (St. Malo, Havre, 

 Basle, Zofingen), and in German and Austro-Hungariau 

 specimens of the var. ridibunda ^ to £ that width is by no 

 means unfrequent. The told is always narrower than the 

 upper eyelid in the vars. chinensis and lessonie. 



In my previous descriptions of the var. ridibunda I have 

 drawn attention to the fact that the dorso-lateral fold, though 

 usually broader than in the other forms, is less prominent ; 

 I should add that it is sometimes so flat that it cannot be 

 traced without the use of a lens, when the pores with which 

 it is studded indicateits course. It has not been pointed out 

 however that these folds are rendered more inconspicuous 

 si ill owing to the spots on the body being disposed quite 

 inespective of them, whilst in the typical form and the vars. 

 lessonce and chinensis they stand out on account of their 

 lighter colour, hardly ever encroached upon by the spots, 

 which may be arranged more or less in relation to them, 

 especially when forming longitudinal bands. When a 

 specimen of the var. ridibunda is seen at a short distance 

 there is usually nothing to reveal the presence of the dorso- 

 lateral folds, which strike the eye in the typical form and 

 the vars. lessonce and chinensis. 



These facts have a bearing on the question of the derivation 

 of the forms which constitute the species R. esculenta, and 

 confirm the view I have held ever since I took up the study 

 of the subject that the var. ridibunda is the most primitive 

 form, out of which the others have been evolved. In a paper 

 recently published * on the derivation of characters in the 

 genus Rana as a whole, the absence of the dorso-lateral fold 

 is considered by me as the primitive condition, and the 

 North American R. catesbiana } in which it is totally absent, 

 is, for this and other reasons, regarded as nearest the hypo- 

 thetical prototype among all the species of Eurasia and 



* Bull. Soc. Zool. France, 1918, p. 111. 



