PROBLEMS AND RELATIONS 5 



tions for climbing and for parachute jumping are especially notable. 

 The animals in mountain streams in all parts of the world have 

 numerous and surprising resemblances in their possession of adhesive 

 apparatus. The faunae of small islands, at considerable distances from 

 the continents, exhibit numerous resemblances in their composition. 

 Beds of moss, from the tropics to the polar regions, are inhabited by 

 communities of animals characterized by the capacity to live in a 

 dormant state for long periods, whether they be protozoans, nematode 

 worms, rotifers, copepods, or tardigrades. These biocoenoses found in 

 a given habitat have analogous communities in similar habitats in the 

 different faunal regions. 



It is only a step from the observation of such groupings of animals, 

 and from the recognition of the fact that they are comparable phenom- 

 ena, to the question of the causes of the appearance of these groups 

 with varying limits, or of the causes which condition the characteristics 

 common to similar habitats. The answer to these problems is sought 

 by causal zoogeography. According as the associations to be studied 

 are homologous or analogous, causal zoogeography studies the reasons 

 for the evident differences in the distribution of the natural groups 

 of animals, or the mutual relations between an environment and its 

 animal population. 



Closely related species of animals will in general have adjacent 

 ranges, since it is to be assumed that the area in which they devel- 

 oped from their common ancestors was the common origin of their 

 distribution. We observe in this respect that related human stocks in 

 general have a continuous distribution. Two factors mutually condition 

 the distribution of a species: the means of dispersal available to the 

 animal, and the barriers opposed by the external world to its progress. 

 As a consequence of the differences in the means of dispersal in dif- 

 ferent natural groups of animals, the external barriers affect the 

 different groups in diverse ways. Water animals are limited in their 

 spread by land, and land animals by water. Land animals are often 

 unable to pass mountain ranges, while flying animals are least af- 

 fected by barriers of any kind. Climate, the lack of suitable food, the 

 presence of more successful competitors or of enemies, may present 

 barriers to the dispersal of any group. 



The means of dispersal remain unchanged through long periods of 

 time; they are as old as the principal subdivisions of the animal 

 kingdom, such as the echinoderms, fishes, insects, or birds, and are, 

 in general, uniform within the group. The more important barriers to 

 dispersal, however, alter with geologic changes in the earth's surface, 

 and these alterations occupy much shorter periods of time than are 



