century, this region was used as an interoceanic passage. The exploration of this region between the ‘North Sea’ and the ‘South Sea’ was, since the conquest of Mesoamerica, a fixed idea of the Spanish monarchy and its representatives. Carlos V made this project one of the priority missions of the explorers whose objective was to put the kingdom of New Spain in communication with that of Peru. Hernán Cortés, in 1520, used this route to transport equipment and supplies across the isthmus from the Gulf of Mexico to his shipyard located on the Pacific coast, near the town of Santo Domingo de Tehuantepec. The advantage lay in the ability to make a good part of the journey by river. Approximately two-thirds of the Isthmus is crossed by the Coatzacoalcos River, which is navigable for approximately 200 kilometers. Cortés saw this river as a means of fluvial communication across the Isthmus itself. During the last third of the sixteenth century, Spanish expeditions arrived at the port of Veracruz, in the Gulf of Mexico, and from there they moved by boat to the mouth of the Coatzacoalcos River. The material necessary to build ships (supplies and tools or provisions) in the shipyards located along the Pacific coast, together with the artillery, were loaded into shallow canoes to navigate upstream, and subsequently transported, on the backs of the indigenous people, from the sources of the Coatzacoalcos River to the Pacific coast, in the Gulf of Tehuantepec.1 Many of these expeditions later moved along the coast to the port of Acapulco, from where they sailed along the western coast of North America, and then set off westwards, seeking a passage to the Philippines and southern China. The advantages of this itinerary would lead to the proposal, as early as the sixteenth century, of an interoceanic canal. However, a series of difficulties resulted in the most feasible option, Panama, eclipsing Tehuantepec as an interoceanic passage. Biogeography The Isthmus of Tehuantepec, a valley otherwise surrounded by montane habitats, has also been noted as an important biogeographical barrier among montane taxa, such as Mexican birds. Population diversification has been observed among not only avian fauna, but other organisms as well, including toads and the Central American river turtle. As a result, the Isthmus presents a case of allopatric speciation wherein a geographic divide gives rise to population divergence and a significant decrease in gene flow. Climate The