Read it through once

One guess as to the origin of Wheat is that the first-named (Mesopotamian sort) is the original wild plant. By cultivation in the rich alluvial valleys of Mesopotamia and Egypt, improved kinds were formed. These have eventually replaced both "spelt" wheat and the wild race, but could only do so when richly-cultivated fields were ready for them. On poor soil and with bad cultivation, "spelt" is said to be even now the most profitable crop.