788 A CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



when a discernible barrier intervened. Wagner's papers on the topic (1868-1886) 

 were collected by his nephew and published in 1889 under the title Die Entstehung 

 der Arten durch rdumliche Sonderung. The importance of this aspect of animal 

 geography is underlined in recent work in genetics and its relation to systematic 

 botany, zoology, and paleontology. I need mention only Julian Huxley's The New 

 Systematics, 1940, Ernst Mayr's comprehensive Systematics and the Origin of 

 Species, 1942, and Allee, et al., The Principles of Animal Ecology, 1949. The 

 phenomena of speciation are of especial interest in older bodies of fresh water 

 and on older oceanic islands as may be seen in Brooks' and Zimmerman's papers 

 cited above. 



Animal Geography of the Sea 



The vast and distinct field represented by the animal geography of the sea is 

 in many respects extremely different from that of the land. Partition into faunal 

 regions and provinces goes back to Schmarda (1853) and was thoughtfully re- 

 viewed by Ortmann in 1896. Barriers are much less likely to be physiographic, 

 are more likely to be related directly to temperature, and when physiographic, 

 may also depend on mere distance. In one important respect, marine zoogeography 

 is complementary to terrestrial zoogeography, namely in the analysis of the 

 phenomenon of interruption of a land connection by the sea, when the new sea 

 passage becomes a highway for dispersal of marine forms as soon as the land 

 highway is broken. The separation of the continents of North and South America 

 during the Tertiary involves a union in this area of the Atlantic and Pacific. The 

 faunal relations thus produced were long ago pointed out by Jordan (1908); 

 they are adequately summarized by Sven Elrnian in his Tiergeographie des Meeres 

 (1935), of which a revised edition in English has recently appeared. Ekman 

 sketches a most convincing picture of the major features of the world pattern 

 of distribution of marine forms, of the operation of the open eastern Pacific and 

 of the Atlantic as barriers to the coastal faunas, and of the historic importance 

 of the Tethys Sea. 



Conclusion 



Animal geography is essentially an evolutionary study. It is only with diffi- 

 culty definable as a separate science. In its descriptive branch it is one of the 

 aspects of general natural history. If the contents of the eleven volumes on verte- 

 brates of Brehm's Tierlehen were rearranged by geographic areas, we should have 

 a comprehensive descriptive animal geography in eleven volumes. Interpretive 

 zoogeography is so intimately related to ecology that it must always be considered 

 as a branch of that synthetic science. Historical zoogeography, finally, is directly 

 dependent on paleontology and thus in turn on geology. These intimate relations 

 with several distinct sciences form the great merit of and the abundant justifica- 

 tion for a science of animal geography. We have now long realized that speciali- 

 zation must be balanced by synthetic sciences that bridge the gaps between the 

 specialties and break down the barriers betw^een the scientists. It is good to turn 

 from small to large problems, to take the long view, to think sometimes in terms 

 of the world as a whole. Only then can we appreciate and interpret our own 



