Young alligators, for example, are guarded by their mothers and owe more to her than they can ever know. She prepares a hatching nursery by scraping together a large mound of leaves, twigs, and fine earth, and upon this mound the eggs are placed about eight inches from the surface. Then the mother digs a hole in the river bank, close by, and here she waits and watches to protect her children. A more advanced form of nursery building is practised by the tortoise, who prepares a sort of nest with considerable care, which she afterwards cunningly conceals. But when once the eggs are safe she shows no further interest in the safety of the nest.[17] Most snakes bury their eggs and then leave them. But a more enduring maternal interest is felt by the mother python; she coils her body around her future family and jealously guards them during the period of incubation, refusing all food and never leaving her duty. A similar guardianship is shown to young crocodiles by their mothers. The home is prepared by digging a deep hole in the sand in which the eggs are placed, and during the period of incubation each mother sleeps in guard above her family. The naturalist Voeltzkow, to prove the reality of one mother crocodile’s solicitude, built a fence around the nest just before the hatching time. Each night on her return, the mother broke down the fence, though each time it was made stronger than the last. Finally the nest was found to be deserted, and then it was discovered that this intelligent and persecuted mother had dug a hole beneath the fence and thence had led her brood away to safety. It is impossible to admire sufficiently such a case as this one, where we see so clearly the driving power of maternal solicitude in quickening the intelligence of even the lowliest mothers. Such cases are, however, few in number out of the 2,600 species of reptiles of whom the majority are unnatural parents.