two most important things to know about Mexico still are patterns of life that existed before coming of white men and changes that were introduced during first generation or two of Spanish period. - Carl O. Sauer, The Personality of 1941 Livestock have occupied Mexican scene since sixteenth-century birth of New Spain, early and pervasively contributing to ecological upheaval of the Columbian exchange (Crosby 1972; Butzer 1992; Turner and others 1995). Spaniards brought livestock both large and small. Along with those cattle, horses, mules, donkeys, sheep, goats, and pigs arrived a suite of ecological institutions that have left decisive imprints on landscape. landscape, reciprocally, has left its imprint on institutions. An association of herders organized transhumant routes by which sheep moved between seasonal pastures along hundreds of kilometers of stonewalled trails, or canadas. estancias, or land grants, gridded landscape league by league, accumulating space for Spaniards and marginalizing natives. pastures burned each long, dry winter, incrementally altering vegetation. Yet despite a deep and broad imprint on Mexican landscape, cattle ranching's sixteenth-century ecological origins have remained obscure. An orthodox literature long emphasized Spain's semiarid plateau - often, specifically, Estremadura - as Old World hearth of New Spain's ranching ecology (Bishko 1952; Brand 1961; Rouse 1977). Supposedly, a fully developed ecology and economy involving mounted herders and transhumance diffused southward with Reconquista into Andalusia and, ultimately, through Seville and across Atlantic to New Spain. More recently, revisionists have combined archival and field research to hypothesize that cattle ranching involving mounted herders reached its Old World apogee along lower Rio Guadalquivir of Andalusia, in seasonally inundated marshes known as Las Marismas, and from there diffused to New Spain (Doolittle 1987; Butzer 1988; Jordan 1993). To date, contrasts between sixteenth-century livestock ecologies of Estremadura and Andalusia provide main support for revisionist hypothesis. Sheep predominated on semiarid plateau, and image of mounted Estremadurans herding cattle across sparse, semiarid grasslands is as much myth as icon. In contrast, cattle predominated in Andalusia, and as floodwaters of Rio Guadalquivir receded each spring, herders drove their stock into Las Marismas [ILLUSTRATION FOR FIGURE 1 OMITTED]. Largely untended during long dry season, isolated among a labyrinth of sloughs, animals became semiferal. Solanos blowing out of Africa parched oak-pine savannas of surrounding hills, which ranchers set ablaze in late summer (Fernandez Ales, Martin, and Merino 1995, 366). As rains returned each October, herders required horses to round up semiferal stock before Guadalquivir flooded and drive them up to fresh regrowth of hill pastures. Andalusian provenance of original conquistadores, four hundred or so who sailed with Cortes, further supports revisionist hypothesis. Nearly a third of conquistadores hailed from provinces bordering Las Marismas - Seville, Huelva, Cadiz - and knew its transhumant pulse (Boyd-Bowman 1964, xli). Having witnessed fecundity of cattle Columbus had introduced into Hispaniola in 1493, those Andalusians extended diffusion to New Spain during sixteenth century (Sauer 1966; Watts 1987). In a few decades, lowland plains along Gulf of Mexico, an environmental homologue to coastal Andalusia, had become domain of vast cattle herds (Simpson 1952; Butzer and Butzer 1995). By 1580 viceroys had granted sixty-two cattle estancias in lowland environs of port of Veracruz alone, and report of alcalde claims 150,000 cows and mares (JGI, XXV-8, f. …