A HISTORY OF THE DEVELOPMENT OF THE QUANTITATIVE STUDY OF VARIATION
ify the distribution of the mammals in such a way that without paleontological researches it would be impossible to recognize the origin of the different fauual elements, the fresh-water faunas have resisted almost unchanged all modifications in the configuration of the continent. The fresh-water fauna is not only older but also much more confiervative than the distribution of the mammals. One of the most striking examples of this is given by the history of Africa. While the characteristic
... characteristic mammals are Neogene immigrants and Lydekker proceeds quite correctly in making Africa a n annex only of the Holarctic region, thus establishing his Arctogsa, with relation to the fresh-water fauna, Africa is a part of Soiltll America, somewhat modified by the Neogene invasion of Cyprinid fishes. If as regards mammals Africa belongs to Arctogaa, with relation to the fresh-water fauna i t belongs to the Archhelenic region. This example demonstrates the absurdity of the present system of covzstr?sction of zoogeographical regions and maps. We can construct maps of the diferent classes and orders but .not at all of the animal kingdom, because the geological history of the diferent groups is quite diferent. When Osborn says that i t is one problem ' to connect living distribution with distribution of past time,' he says only what had been the leading idea of Wallace and of Erlgler in their eminent works on ZOOand phytogeography, but when he continues ' and to propose a system which will be in harmony with both sets of facts,) be proposes a problem just as contradictory as would be the construction of descriptions and figllres reforring a t the same time to egg, larva, nympha and imago of an insect. The works on zoogeography ' are almost exclusively discussions of the distribution of mammals and birds, and the few words spent on otller classes are only ornamental supplements. A wrong method cannot give valid results. For the exploration of the zoogeographical relations and regions of the beginning of the Tertiary and of the preceding Mesozoic epoch it is necessary to study and to discuss the moreancient classes and, as I have insisted for ten years, principally the fresh-water fauna.