To this we must add the fact that Egyptian civilization was especially immobile: its development into castes is but one among many evidences that it was the very opposite of a civilization developed rapidly.
As to the length of the period before the time of Mena, there is, of course, nothing exact.

Manetho gives lists of great personages before that first dynasty, and these extend over twenty-four thousand years.
Bunsen, one of the most learned of Christian scholars, declares that not less than ten thousand years were necessary for the development of civilization up to the point where we find it in Mena's time.

No one can claim precision for either of these statements, but they are valuable as showing the impression of vast antiquity made upon the most competent judges by the careful study of those remains: no unbiased judge can doubt that an immensely long period of years must have been required for the development of civilization up to the state in which we there find it.
The investigations in the bed of the Nile confirm these views.

That some unwarranted conclusions have at times been announced is true; but the fact remains that again and again rude pottery and other evidences of early stages of civilization have been found in borings at places so distant from each other, and at depths so great, that for such a range of concurring facts, considered in connection with the rate of earthy deposit by the Nile, there is no adequate explanation save the existence of man in that valley thousands on thousands of years before the longest time admitted by our sacred chronologists.
Nor have these investigations been of a careless character.

Between the years 1851 and 1854, Mr.Horner, an extremely cautious English geologist, sank ninety-six shafts in four rows at intervals of eight English miles, at right angles to the Nile, in the neighbourhood of Memphis.