PROBLEMS AND RELATIONS 7 



omy" of plants and animals. Ecological zoogeography views animals 

 in their dependence on the conditions of their native regions, in their 

 adaptation to their surroundings, without reference to the geographic 

 location of this region, whether in America or Africa, the northern or 

 the southern hemisphere. This phase of zoogeography may also pro- 

 ceed either from a geographic unit or from the animal itself. The 

 questions in the former case concern the requirements created by the 

 habitat conditions of a given area for the structure and habits of its 

 inhabitants; the modifications in appearance and habits undergone 

 by the animal population in adaptation to the given conditions; and 

 the selective operation of the habitat requirements on the composi- 

 tion of the fauna. If a specified animal be made the starting point 

 for the studies to be pursued, the questions concern the anatomical 

 and physiological characters which fit it to its surroundings, and 

 enable it to compete successfully in the struggle for existence; the 

 peculiarities due to the direct influence of the surroundings; and the 

 reasons for its failure to spread into other environments. In this way 

 an explanation may be found for the convergent evolution of different 

 animals under the influence of similar environmental conditions. 



The results of historic and ecologic studies in zoogeography are 

 mutually supplementary; but on account of the differences in their 

 methods, the relative value of their conclusions is very unequal. The 

 historic mode of approach deals primarily with the geological history 

 of the earth and with the phylogeny of the animal kingdom. As in 

 human history, the events of geologic history and of animal evolution 

 are never twice exactly alike, and it has been fruitless to seek in them 

 for universal causal connections, as was attempted by Reibisch and 

 Simroth with their pendulation theory of climate and by Eimer with 

 his orthogenetic theory of species formation. The reconstruction of 

 such past events is consequently uncertain, and the number of errone- 

 ous conclusions excessively large. The abundance of incompatible 

 hypotheses for former land connections between continents has been 

 shown graphically by Handlirsch, 4 who figures all the supposed land 

 bridges of Cretaceous and Tertiary time on the same maps; scarcely 

 a bit of ocean has escaped the supposition of having formerly been 

 occupied by land. In an important work on the polyphyletic origin 

 of the large terrestrial birds of the southern hemisphere, R. Burck- 

 hardt 5 has brought out the mistakes produced, in another direction, 

 by false phylogenetic premises. 



The case is quite different with the ecological method. Ecology 

 deals with the conditions and phenomena of the present, which are 

 subject to analysis and repeated test. Instead of being concerned with 



