As the climate improved the lands swept bare once again became inhabitable, and there was a recolonisation by movements from the south and from the east. We shall indicate later how man himself came from the south and the east to colonise the west and north, but his movements were only part of a great series which included also those of plants and animals. CHAPTER IV CLIMATE AND WEATHER To the superficial observer those daily variations in the atmospheric conditions in any one locality which we sum up under the term weather, may appear to occur without order or regularity, but detailed quantitive study soon shows that even British weather displays constancy in its irregularity. The existence of such basal constancy, indeed, lies at the root of all intelligent utilisation of the soil. The irresponsible amateur gardener may lightheartedly assume that a particular spring will be "early," but the professional is not easily induced to abandon his rule that such and such operations must not be undertaken before certain fixed dates. The farmer, if he is to avoid bankruptcy, must know within what limits the first autumn frost is likely to make its appearance, and when the last spring one may be expected. Collective experience, then, whether expressed in the meteorologist's figures or in a less accurate form, leads us to the conclusion that for every locality on the earth's surface there is a certain fixed average succession of weather, which we sum up in the term _climate_. In the case of both climate and weather our knowledge may be summed up in such general terms as "wet" or "dry," "warm" or "cold," and so forth, or we may borrow the meteorologist's notations, and express the facts in degrees of temperature, inches of rainfall and of pressure, percentages of humidity, and so on. But it should be understood that such figures can be used by the geographer with justification only when he is himself aware, and can assume that his audience is aware, of the significance of the figures in connection with the processes of erosion and the phenomena of life. To say that the mean January temperature of a particular place is 30° F., is only a convenient shorthand way of saying that in this place in winter plant life is arrested, water is ice-bound, and most animals sleep or migrate. In other words, the use of the figures assumes a certain knowledge of biology and of physics on the part of the audience. We do not propose here to treat either climate or weather with any fullness, for there is a volume in the series specially devoted to these and kindred subjects. All that will be attempted, therefore, is to discuss one or two important climates with the object of considering later their respective effects on the distribution of other phenomena on the surface of the globe. This is the more worth doing in that the subject is one which has had a great deal of attention devoted to it in recent years.