HABITS. 57 



VII. Habits. 



Frogs and toads were classified by the earlier authors 

 into terrestrial, aquatic, and arboreal. It would be 

 difficult now, nay impossible, to draw any sharp demar- 

 cation line between tbe three types, and all we can do 

 is to emphasise the differences which exist between the 

 extreme forms. Thus by opposing the extremes we 

 can show examples among the European representa- 

 tives of terrestrial and aquatic (Alytes obstetricans — 

 Eana esculenta) t fossorial and scansorial (Pelobates — 

 Hyla), saltatorial and cursorial (Rana agilis — Bufo 

 calamita), diurnal and nocturnal {Rana esculenta — 

 Alytes obstetricans) types, which are, however, con- 

 nected by insensible gradations. These adaj)tations 

 are revealed by certain structural characters, such as 

 the development of the web between the toes to facili- 

 tate natation, the great development of the inner 

 metatarsal tubercle to assist in burrowing, the adhesive 

 digital disks for climbing, the relative length of the 

 hind limbs for jumping, and the shape of the pupil, 

 whether round or horizontal or vertical, as indicative 

 of more or less diurnal or essentially nocturnal habits. 

 But here and there we meet with exceptions; such 

 are, for instance, the fully-webbed toes of Pelobates, 

 which spends but a short time of the year in the 

 water; the shovel-shaped metatarsal tubercle of liana 

 esculenta, var. lessonse, which does not burrow; or the 

 similarity in shape of the pupil in Rana and Bufo, 

 which yet differ so much in their habits. Still, on the 

 whole we may roughly incorporate the species outside 

 the breeding season into the following categories: 



Diurnal and aquatic : Rana esculenta, Discoglossus, 

 Bomhinator. 



Diurnal and terrestrial : Rana temporaria and allies. 



Nocturnal and terrestrial : Bufo, Pelodytes, Alytes. 



Nocturnal and fossorial : Pelobates. 



Nocturnal and arboreal : Hyla. 



That most, though not all, are aquatic during the 



