but it cannot always have been enforced, for the tombs of the Apis bulls have been discovered in the present century, and from the inscriptions on them it appears that in the twenty-second dynasty two bulls lived more than twenty-six years.(227) We are now in a position to hazard a conjecture—for it can be little more—as to the meaning of the tradition that Virbius, the first of the divine Kings of the Wood at Aricia, was killed by horses. Having found, first, that spirits of vegetation are not infrequently represented in the form of horses;(228) and, second, that the animal which in later legends is said to have injured the god was sometimes originally the god himself, we may conjecture that the horses by which Virbius was said to have been slain were really embodiments of him as a deity of vegetation. The myth that Virbius had been killed by horses was probably invented to explain certain features in his cult, amongst others the custom of excluding horses from his sacred grove. For myth changes while custom remains constant; men continue to do what their fathers did before them, though the reasons on which their fathers acted have been long forgotten. The history of religion is a long attempt to reconcile old custom with new reason; to find a sound theory for an absurd practice. In the case before us we may be sure that the myth is more modern than the custom and by no means represents the original reason for excluding horses from the grove. From the fact that horses were so excluded it might be inferred that they could not be the sacred animals or embodiments of the god of the grove. But the inference would be rash. The goat was at one time a sacred animal or embodiment of Athene, as may be inferred from the practice of representing her clad in a goat-skin (_aegis_). Yet the goat was neither sacrificed to her as a rule, nor allowed to enter her great sanctuary, the Acropolis at Athens. The reason alleged for this was that the goat injured the olive, the sacred tree of Athene.(229) So far, therefore, the relation of the goat to Athene is parallel to the relation of the horse to Virbius, both animals being excluded from the sanctuary on the ground of injury done by them to the god. But from Varro we learn that there was an exception to the rule which excluded the goat from the Acropolis. Once a year, he says, the goat was driven on to the Acropolis for a necessary sacrifice.(230) Now, as has been remarked before, when an animal is sacrificed once and once only in the year, it is probably slain, not as a victim offered to the god, but as a representative of the god himself. Therefore we may infer that if a goat was sacrificed on the Acropolis once a year, it was sacrificed in the character of Athene herself; and it may be conjectured that the skin of the sacrificed animal was placed on the statue of the goddess and formed the _aegis_, which would thus be renewed annually. Similarly at Thebes in Egypt rams were sacred and were not sacrificed.