works, such as the building of churches, although the kings did not always so employ it.[37] [Sidenote: Advance of the middle class.] The same causes which had conduced to the development of the middle class in the preceding era were accentuated to procure a corresponding advance in this,--such as the increase in population, the growth of industry, commerce, and agriculture, the freedom of the servile classes, the prominence of the jurisconsults and secondary nobility, or _caballeros_ (proceeding usually from the towns, and living there allied with the middle class against the greater nobles), and the great political importance which the towns acquired. The basis of the middle class was the town, partisan of the centralizing, absolutist tendency of the kings so far as it related to the nobles and clergy, but strenuously insistent on the retention of its own local charter. The middle class had control of production and was the nerve of the state, but was virtually the only element to pay taxes, despite the fact that the great bulk of territorial wealth was in the hands of the nobility and the church. The term “middle class” began to refer more and more clearly to the wealthier, free, but untitled element, for the laboring class became more prominent in the towns, sharing in the charter privileges of their richer neighbors, but with certain limitations on their economic liberty. There was no social conflict of consequence between the two classes, however, for the laborers were not yet very numerous, and the evils of their situation were not so great as they later became, besides which, self-interest united them with the middle class against the nobles and clergy. Such strife as there was between them was of a political, and not of a social, character. The so-called popular element of the _Cortes_ represented the middle class only. The practice of forming leagues (_hermandades_) of towns and _caballeros_ against the abuses of the higher nobility was much indulged in, for it was not safe to rely solely on the king. The victory in the end lay with the towns, although they were far from obtaining their specific aims at this time. Nevertheless, the fourteenth century was characterized by the transformation of society from its earlier basis of chivalry and war, when the scene had been laid in the castles of the country, to the bourgeois life of the towns, devoted to industry and commerce. [Sidenote: Improved basis of rural society.] [Sidenote: Slavery.]