6 INTRODUCTION. 



shade. There is no doubt that difference of habitat influences the 

 colour; but this may again be modified by light ^^ as has been 

 established in the case of fish by direct observation-. Apparent 

 varieties may thus occur. 



In frog-tanks such diversities of colour may not unfrequently be 

 observed in the same individual^ as for example when the lower part 

 of the body immersed in muddy water is dark, while the part above 

 the water is bright. That the process of casting the skin exercises 

 an influence on the brightness of the colouring is certain, yet there 

 are, as von Wittich has correctly remarked, other alterations of 

 colour which are in no way connected with this process, and are 

 evidently more of a pathological nature ; such as when the frog 

 assumes a dirty green spotted appearance, the green fading more 

 and more, until all the patches which are usually green appear 

 of a dirty greyish-brown with a faint bronze shimmer. According 

 to this author these changes are most readily brought about by 

 starvation. The dark colour which frogs exhibit after hibernation 

 is perhaps to be ascribed to the co-operation of several of the causes 

 mentioned above. 



The usual colouring of healthy animals is as follows : the back 

 is bright green with three golden yellow longitudinal stripes, one 

 median and two lateral, and a number of irregular brown or black 

 stripes of approximately miiform width : on the head are a pair of 

 black stripes which pass from the angles of the eyes across the nares 

 to the tip of the nose ; now and then the tympanic membrane and 

 surrounding parts have also a black patch, as in B. iemporana : 

 another black stripe is found on the anterior surface of the arm, in 

 the region of the shoulder : and on the thighs are black, yellow, and 

 white mottlings. The whole of the under-surface is white or 

 yellowish. At times the yellow stripes of the back are wantmg or 

 are indistinct. It has already been mentioned that many varieties 

 may occiir ; and these have in all probability given rise to the de- 

 scriptions of reputed new species, such as R. maritima, Risso, found 

 in South Europe ; R. alpina, Risso, found in the high -lying Alpine 

 lakes ; R. hispanica of Fitzinger and Bonaparte, and R. calcarata of 

 Michahelles, the last three of which certainly cannot be retained. It 



' Lister, On the Cutaneous Pigmentary System of the Frog. Phil. Trans., 1857, 

 p. 627. 



^ Agaosiz et Vogt, Histoire naturelle des poissons d'eau douce (Neuchatel, 1839), 

 PI. IV, mention that the colour of trout is very variable and that in shaded and 

 deep-lying brooks and rivers a variety is found which is black. 



