However, this identification of Isis with the cow, like that of Osiris with the bulls Apis and Mnevis, may be only an effect of syncretism. But whatever the original relation of Apis to Osiris may have been, there is one fact about the former which ought not to be passed over in a disquisition on the custom of killing a god. Although the bull Apis was worshipped as a god with much pomp and profound reverence, he was not suffered to live beyond a certain length of time which was prescribed by the sacred books, and on the expiry of which he was drowned in a holy spring.(139) The limit, according to Plutarch, was twenty-five years;(140) but it cannot always have been enforced, for the tombs of the Apis bulls have been discovered in modern times, and from the inscriptions on them it appears that in the twenty-second dynasty two of the holy steers lived more than twenty-six years.(141) (M30) To prevent misunderstandings it may be well to add that what I have just said as to the stratification of three great types of religion or superstition corresponding to three great types of society is not meant to sketch, even in outline, the evolution of religion as a whole. I by no means wish to suggest that the reverence for wild animals and plants, the reverence for domestic cattle, and the reverence for cultivated plants are the only forms of religion or superstition which prevail at the corresponding stages of social development; all that I desire to convey is that they are characteristic of these stages respectively. The elements which make up any religious system are far too numerous and their interaction far too complex to be adequately summed up in a few simple formulas. To mention only a single factor of which I have taken no account in indicating roughly a certain correspondence between the strata of religion and of society, the fear of the spirits of the dead appears to have been one of the most powerful factors, perhaps, indeed, the most powerful of all, in shaping the course of religious evolution at every stage of social development from the lowest to the highest; and for that very reason it is not specially characteristic of any one form of society. And the three types of religion or superstition which I have selected as characteristic of three stages of society are far from being strictly limited each to its corresponding step in the social ladder. For example, although totemism, or a particular species of reverence paid by groups of men to wild animals and plants, probably always originated in the hunting stage of society, it has by no means been confined to that primitive phase of human development but has often survived not only into the pastoral but into the agricultural stage, as we may see for example by the case of many tribes in Africa, India, and America; and it seems likely that a similar overlapping of the various strata takes place in every instance. In short, we cannot really dissect the history of mankind as it were with a knife