America, with the Polynesian and Indonesian cultural regions, shows the correlation and the systematic discrepancy in time between the rate, range and magnitude of the advance in tillage on the one hand and of the primary mechanic arts on the other hand. When this culture was interrupted it had, in the mechanical respect, reached an advanced neolithic phase at its best; but its achievements in the crop plants are perhaps to be rated as unsurpassed by all that has been done elsewhere in all time.[39] In the primary mechanic arts this cultural region had in the same time reached a stage of perfection comparable at its best with pre-dynastic Egypt, or neolithic Denmark, or pre-Minoan Crete. The really great advance achieved was in the selection, improvement, use and cultivation of the crop plants; and not in any appreciable degree even in the mechanical appliances employed in the cultivation and consumption of these crops; though something considerable is to be noted in this latter respect in such inventions as the mandioca squeezer and the metate; and great things were done in the way of irrigation and road building.[40] But the contrast, for instance, between the metate and the contrivances for making paper bread on the one side, and the technologically consummate corn-plant (maize) on the other, should be decisive for the point here in question. The mechanic appliances of corn cultivation had not advanced beyond the digging stick, a rude hoe and a rudimentary spade, though here as well as in other similar connections the local use of well-devised irrigation works, terraced fields,[41] and graneries is not to be overlooked; but the corn itself had been brought from its grass-like ancestral form to the maize of the present corn crop. Like most of the American crop plants the maize under selective cultivation had been carried so far from its wild form as no longer to stand a chance of survival in the wild state, and indeed so far that it is still a matter of controversy what its wild ancestor may have been.