S TUDIES that seek to explain the present distribution of animal forms must draw on a wide and varied field of evidence. Some of the evidence, notably that provided by the paleontologists, is direct and generally capable of reliable interpretation; but rarely, except for some groups of land mammals, is the fossil evidence sufficient. More commonly, paleontology can provide only the framework, perhaps one or two major clues, or even just a tiny piece of the whole picture. The most successful ventures into the field of historical zoogeography have been made by paleomammalogists who have studied particularly the groups living in relatively large numbers in habitats where sedimentation is most rapid. Thus the historical geography of such plains-dwelling groups as the horses and the elephants is comparatively well known, but forms that inhabit mountains, forests, or seas are represented only fragmentarily in the fossil record. For these latter groups other evidence must be invoked, and it is provided by the taxonomy, physiology, ecology, and distribution of the existing forms. This evidence, although easier to accumulate, is not capable of such reliable interpretation as that provided by paleontology, but it must always be explained and often provides the only clues available. A third, more nebulous, category of evidence derives from our knowledge of past climates and past distributions of land and sea. At present, however, this knowledge is so uncertain that it is clearly dangerous to place too much reliance on it; at the same time, it should not be disregarded, since it can often indicate possibilities and probabilities and provide a set of limits within which the correct solution to any given problem may be found. Finally, it is necessary to keep in mind present trends of thought in the fields of genetics and evolution, because, although there is anything but unanimity of opinion within these fields, the most generally accepted concepts will have an important bearing on possible zoogeographical conclusions. Individual bits of evidence culled from all four sources paleontological, neozoological, paleogeographical, and evolutionary may often appear