784 A CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



have included an overlooked fundamental error. How could a theory founded on 

 so inadequate and one-sided an explanation of the facts have been so long main- 

 tained and served so well as a guide for the exploration of North America? It 

 is simply that the facts of the distribution of plant and animal life in a pattern 

 related to the transverse climatic zones on a continental scale and to the altitude 

 zones in mountains are quite independent of attempted explanations. It can 

 scarcely be too much emphasized that animal life is dependent on plant life 

 in far more ways than a diagram of food relations of plants and animals indicates. 

 Animals live in a plant matrix, whose importance is measured somewhat by the 

 difference in the tropical forest of the order of many thousands to one, and is still 

 very great in the most densely populated savanna. I find this set forth force- 

 fully by Alexander von Humboldt in his Kosmos (1845), and as early as 1808 in 

 Ansichien der Natur. "Aspects of Nature" is in fact the key to the discrimination 

 of the biotic areas for which we are in search; they prove to be precisely those 

 vegetational areas that are visibly distinguishable in the landscape. 



The pattern of life zones, which is so conspicuously transcontinental in the 

 open tundra and coniferous forest of northern North America, becomes more 

 and more obscure in the southern half of the continent, and the Carolinas and 

 California are radically distinct in both vegetation and fauna. The governing 

 factors toward the south become more clearly those of humidity and rainfall 

 instead of temperature, and there is then the further increase in complexity of the 

 historical factors. We may accordingly turn to a much more complicated partition 

 of the continent into biotic provinces. Work in this direction is exemplified in 

 papers by Shelford and Pitelka, and especially by the little book by Lee R. Dice, 

 The Biotic Provinces of North America (1943). 



Various papers by C. C. Adams deal with problems of animal geography 

 from an ecological standpoint, and with his assistant at the University of Michi- 

 gan, Alexander G. Ruthven (subsequently director of the University Museum), 

 he focused interest on ecology in museum field work. His paper on the dispersal 

 of the biota of the southeastern United States (1903) well illustrates these 

 interests. Through Ruthven he left a permanent stamp on the Museum of Zoology 

 of the University of Michigan, and Ruthven 's lectures on zoogeography influenced 

 a generation of students. 



Distributions of the past were of course quite as much determined by the 

 total ecology as those of the present; but we can only discern those paleoeco- 

 logical factors with difficulty and with much more critical attention to the modes 

 of occurrence of fossils than has been thought necessary hitherto. This is, how- 

 ever, a fully recognized direction of effort in geology, and we have in progress 

 a cooperative Treatise on 3Ia7'ine PaleoecoJogy of monumental proportions. Paleo- 

 climatology, more specifically of the land areas, has had long attention, and is 

 summarized in special works such as Brooks' Climate Through the Ages (2d ed. 

 1949). 



Two papers may be cited as exemplars of paleoclimatology applied to zoo- 

 geographic studies. The first of these is Alfred Nehring's classic Ueher Tundren 

 und Steppen der Jetzt- und Vorzeit (1890). Nehring analyzes the fauna of the 

 existing northern tundra and of the existing Asiatic steppes (the semi-arid 

 grasslands), and then examines fossil finds of these tundra and steppe animals in 

 Europe in relation to the advances and retreats of the continental glaciers of the 



