Well-authenticated instances of different species interbreeding are unknown, but specimens intermediate between Vipera berus and V. aspis, and between V. berus and V. ammodytes, have been assumed, with much probability, to be hybrids.
CHAPTER X
HABITS
Snakes may be grouped, according to their mode of life, in five principal categories, gradually merging into each other, or two of them not infrequently found combined in one and the same species. These categories are:—Ground-snakes, Sand-snakes, Burrowing-snakes, Tree-snakes, and Water-snakes.
Ground-snakes may be defined as living above ground, and only occasionally climbing bushes or entering the water. Among European genera, Coronella and Vipera are perfect examples of this type, whilst Coluber and Zamenis approach the Tree-snakes in often ascending bushes, or even trees.
Sand-snakes are adapted for living on loose sand, in which they seek concealment. Such are Lytorhynchus and some Psammophis among the Colubridæ, Cerastes among the Viperidæ. Eryx connects this category with the next.
Burrowing-snakes live chiefly underground, and often have the visual organ atrophied in consequence, as in Typhlops; all the Typhlopidæ, Glauconiidæ, and Uropeltidæ, belong to this category; the Viperid Atractaspis is also a burrowing type.
Tree-snakes spend the greater part of their life on bushes or trees. Corallus among the Boidæ, Dendrophis and Dendraspis among the Colubridæ, Atheris and various species of Lachesis among the Viperidæ, may be quoted as examples.
Of Water-snakes, some are exclusively aquatic, like the marine Hydrophiinæ and the typical Acrochordinæ (Acrochordus, Chersydrus) and Homalopsinæ (Hipistes, Herpeton). Chersydrus and Hipistes occur in the sea as well as in fresh water. Many species of Tropidonotus (T. tessellatus and T. viperinus in Europe), as well as the genera Helicops, Grayia, Boulengerina, etc., among the Colubridæ, Eunectes among the Boidæ, Ancistrodon piscivorus among the Viperidæ, are chiefly but not exclusively aquatic.