To-day, the faunas of the land, fresh waters and shallow seas are not uniformly distributed over the world. For example, each of the great land-masses has a fauna, which taken as a whole, is peculiar to it, and as the present distribution reflects the history of the individual forms, it is to be expected that similar zoo geographical regions should be recognisable at every geological period. In fact it is found from the evidence of mammals that in Oligocene times North America, Mongolia and Europe, Africa, Australia and South America formed independent provinces. Some facts of the distribution of ammonites suggest that in Cretaceous times a similar regional arrangement occurred in the seas. Owing to the difficulty of being certain that the observed faunal differ ences in this and similar cases do not really depend on the absence of deposits of identical age, it is rarely possible to determine the limits or indeed the real existence of palaeo-zoo-geographical regions in the sea.
It has often been stated that in past times the distinction be tween the climates of the tropical and temperate zones was far less pronounced than it is to-day. This statement is based on the fact that certain floras and faunas, the marine fauna of the Lower Carboniferous, and the Rhoetic flora, for example, spread uni formly over the world, and that their remains may be found in differently in the tropics and within the Arctic circle. It is
difficult to conceive of any circumstances which could have greatly modified the existing arrangement, and the wide distribution of certain faunas must be accounted for either on a basis of inade quate discrimination of species, or on a greater adaptability of the animals concerned to temperature changes. There are, in fact, certain cases where the distribution of fossil animals can only be accounted for by the existence of such temperature belts. Thus comparison of the fauna found in the Upper Marine series of Permian age in New South Wales, which from the occurrence of isolated glacial blocks on it, is known to have been formed under conditions analogous to those now existing in the Antarctic seas, with the Permian faunas of the Himalayas, which cover the same period shows conclusively that we are here dealing with the results of temperature differences. The two faunas are completely dis similar and it is most improbable that this can be accounted for by varying facies.