Thus from the north to the south of Argentina there is a complete contrast in the way in which the controlling forces of the landscape are distributed. In the north the moist winds come from the east; the rains lessen as they pass westward. The clays, capped with black soil, of Buenos Aires are aeolian deposits, brought by the wind from the desolate steppes which close the Pampa to the west, fixed and transformed by the vegetation of a moister region. In the south, on the contrary, the rains come from the Pacific, and the fluvio-glacial alluvial beds of the Patagonian tableland are evidence of copious reserves of moisture in the Andes; but the arid climate in which the waters have left them has made its mark upon their surface. This diversity of the physical environment is only fully brought out by colonization. It is colonization, the efforts and attempts of human industry to adjust agricultural or pastoral practices to the natural conditions, which enable us to assign the limits of the natural regions. In this differentiation it is essential to notice the historical element. The introduction of new crops gives a geographical meaning, which had hitherto escaped observation, to climatological limits such, for instance, as the line of 400 millimetres of rainfall which is the western frontier of the region of cereals. These limits of crops remain uncertain for a time, then experience and tradition gradually fix them. They always keep a certain elasticity, however, advancing or receding according as the market for the particular produce is favourable or unfavourable. Improvement in the methods of exploiting the soil--the adoption of better agricultural machinery, dry farming, etc.--usually leads to the extension of the sphere of a particular type of colonization, as it enables this type to overcome some natural obstacle which restricted its expansion. Sometimes, however, it brings to light a new obstacle and creates a new geographical limit. To this category belongs the northern limit of the belt of selective breeding, which slants across the plain of the Pampas from the Sierra de Cordoba to the Parana. The more or less degenerate cattle of the natives had spread over the whole of the South American continent, except the tropical forests, since the seventeenth century, adapting themselves easily to very different climatic conditions, from the Venezuelan _llanos_ to the _sertao_ of Bahia and the plains of Argentina. But pedigree animals, more valuable and more delicate, introduced on to the Pampas fifty years ago, are not able to resist the malady caused by a parasite called the _garrapate_. Hence the southern limit of the _garrapate_ suddenly became a most important element in the economic life of the Republic. It would lose its importance if we discovered a serum that would give the animals immunity against Texas fever.