Opportunities of study and of observation afforded in menageries and zoological gardens at the present day have caused the term _viper_ as relating to gestation to be discarded, or many non-venomous snakes must be included, thus overthrowing all our notions of vipers. As was shown in the preceding chapters, the name is now associated with dentition. German and French ophiologists affirm that the three distinctions of _oviparous_, _viviparous_, and _ovoviviparous_ are founded on no other ground than the greater or less development of the fœtus at the time of deposition. The nature of the egg-covering or ‘shell’ has also to do with this. In eggs which take a longer time to mature or to ‘hatch,’ the external covering is thicker and more leathery; in those which are hatched either before or on deposition, the shell is thinner, more membranous. Always, however, there is a calcareous element in the shell, and the eggs are generally, _but not invariably_, linked together. Heat and moisture are essential to the hatching of eggs. When at liberty the snake selects some spot among decaying leaves, or in a manure heap where decomposition produces sufficient warmth. In the tropics, where the sun’s rays alone suffice, a soft moist bed is more easily found, and here it is that immense broods are produced. The period of gestation can scarcely be pronounced upon with certainty. It depends not only on the size of the snake, but on the degree of warmth that can be enjoyed as an assistant to mature the eggs. Schlegel mentions three or four months from copulation to the laying of eggs in the species indigenous to France. But as other circumstances combine to cause variations in these periods, it is very unsafe to fix upon the precise time of gestation. Says Rymer Jones, ‘Reptiles do not sit (_sic_) upon their eggs, hence the latter have only a membranous envelope. In many of the reptiles which lay eggs, especially the _Colubri_ (colubrine snakes), the young one is already formed and considerably advanced in the egg at the moment when the mother lays it; and it is the same with those species which may at pleasure be rendered viviparous by retarding their laying.’[129] The latter words are traced to Cuvier, and prove that this most remarkable power has long been recognised. In the first few words of the above, Jones spoke of reptiles generally from toads to turtles; with the latter, soft eggs would certainly fare badly did they attempt to incubate them. Still the term ‘reptiles’ is misleading, because, as is now well known, some snakes do incubate, and some lizards are suspected of doing the same. Even our common ring snake has been found coiled upon her eggs.