In view of these facts it may be inferred that dry-farming is an ancient practice. It is improbable that intelligent men and women could live in Mesopotamia, for example, for thousands of years without discovering methods whereby the fertile soils could be made to produce crops in a small degree at least without irrigation. True, the low development of implements for soil culture makes it fairly certain that dry-farming in those days was practiced only with infinite labor and patience; and that the great ancient nations found it much easier to construct great irrigation systems which would make crops certain with a minimum of soil tillage, than so thoroughly to till the soil with imperfect implements as to produce certain yields without irrigation. Thus is explained the fact that the historians of antiquity speak at length of the wonderful irrigation systems, but refer to other forms of agriculture in a most casual manner. While the absence of agricultural machinery makes it very doubtful whether dry-farming was practiced extensively in olden days, yet there can be little doubt of the high antiquity of the practice. Kearney quotes Tunis as an example of the possible extent of dry-farming in early historical days. Tunis is under an average rainfall of about nine inches, and there are no evidences of irrigation having been practiced there, yet at El Djem are the ruins of an amphitheater large enough to accommodate sixty thousand persons, and in an area of one hundred square miles there were fifteen towns and forty-five villages. The country, therefore, must have been densely populated. In the seventh century, according to the Roman records, there were two million five hundred thousand acres of olive trees growing in Tunis and cultivated without irrigation. That these stupendous groves yielded well is indicated by the statement that, under the Caesar's Tunis was taxed three hundred thousand gallons of olive oil annually. The production of oil was so great that from one town it was piped to the nearest shipping port. This historical fact is borne out by the present revival of olive culture in Tunis, mentioned in Chapter XII.