«Simpler form of open-field husbandry under the tribal system.» But it must not be forgotten that side by side with this manorial open-field system we found an earlier and simpler form of open-field husbandry carried on by the free tribesmen and taeogs of Wales. This simpler system described in the Welsh laws and the 'triads' seemed to be in its main features practically identical with that described also in the _Germania_ of Tacitus. It was an annual ploughing up of fresh grass-land, leaving it to go back again into grass after the year's ploughing. It was, in fact, the agriculture of a pastoral people, with a large range of pasture land for their cattle, a small portion of which annually selected for tillage sufficed for their corn crops. This is clearly the meaning of Tacitus, '_Arva per annos mutant et superest ager._' It is clearly the meaning of the Welsh 'triads,' according to which the tribesman's right extended to his 'tyddyn,' with its corn and cattle yard, and to _co-aration of the waste_. «Three-field system produced by a three-course rotation of crops.» Nor can there be much mystery in the relation of these two forms of open-field husbandry to each other. In both, the arable land is divided in the ploughing into furlongs and strips. There is co-operation of ploughing in both, the contribution of oxen to the common team of eight in both, the allotment of the strips to the owners of the oxen in rotation, [p370] producing the same scattering of the strips in both. The methods are the same. The difference lies in the application of the methods to two different stages of economic growth. The simple form is adapted to the early nomadic stage of tribal life, and survives even after partial settlement, so long as grassland is sufficiently abundant to allow of fresh ground being broken by the plough each year. The more complex and organised form implies fixed settlement on the same territory, the necessity for a settled agriculture within a definite limit, and the consequent ploughing of the same land over and over again for generations. The _three-field_ system seems to be simply the adaptation of the early open-field husbandry to a permanent three-course rotation of crops. «The yard-land the mark of serfdom.» But there is a further distinguishing feature of the English three-field system which implies the introduction of yet another factor in the complex result, viz. the _yard-land_. And this indivisible bundle of strips, to which there was always a single succession, was evidently the holding not of a free tribesman whose heirs would inherit and divide the inheritance, but of a serf, to whom an outfit of oxen had been allotted. In fact, the complex and more organised system would naturally grow out of the simpler form under the two conditions of _settlement_ and _serfdom_.