308 PRTNCTI'LES OF AXnfAL IIIOLOCIY 



directions it is limited l>y unfavorable conditions either throughout its 

 life or for some time. In oth(u- directions it extends its range. Anywhere 

 within its range new types of individuals may arise through the process of 

 evolution. These new types may be fitted to occupy new regions, and 

 if they are formed near the limits of the range they may find opportunity 

 to spread into areas which arc inaccessible to the unaltered members 

 of the species. Thus may arise recognizably distinct forms coincident 

 in range with certain environmental conditions. If particular forms, or 

 the individuals of a single form, are accidentally (or possibly by sporadic 

 migration) transferred across barriers the distribution of the group be- 

 comes discontinuous. If these processes have been going on for a long 

 time, that is, if the common ancestors of a group of forms existed long 

 ago, the range may have had time to become very extensive, or its dis- 

 continuity very marked. If, contrariwise, the ancestors were compara- 

 tively recent, the range is likely to be much smaller. For this reason, 

 groups that have diverged far enough to have attained the rank of families 

 are on the whole more widespread than those so nearly allied as to be con- 

 sidered genera. Should the environment become altered within a given 

 range, the occupying form might be driven from it or destroyed. If the 

 environment in a region adjoining a range should change in a favorable 

 manner, the range might be extended at that point without any altera- 

 tion on the part of the animals. 



The distribution of animals is inferred to be in harmony with this 

 method, which involves, it will be noted, the factors of migration, evolu- 

 tion, physiological and morphological dependence upon the environment, 

 the diversity and changeableness of the earth's surface, and extinction; 

 and in this manner are explained the differences in geographical position, 

 differences in size of range, differences in the continuity of range and the 

 fact that ranges are at first continuous, differences in physical and bio- 

 logical conditions which characterize the ranges of different forms, and 

 the geographical proximity of apparently related forms. 



Methods and Aims of Zoogeography. — The principal data of zoogeog- 

 raph}^ are museum specimens and i)ublished records of the occurrence 

 of the forms. Modern museums endeavor to preserve specimens with 

 detailed locality data and to obtain material from every part of the range 

 of each species. From the museum specimens and records (and the stu- 

 dent must usually obtain them from as many museums as possible) the 

 ranges are described or mapped. The descriptions are usually brief 

 summaries in which the localities on the outskirts of the known range are 

 given. For example Stejncger and Barbour give the range of the frog 

 Pseudacris ornata as ''South Carolina to Florida, west to Texas," and 

 that of Pseudacris feriarum as "Eastern United States, west to Illinois." 

 When the distribution is mapped this is usually done by plotting the 

 known localities or by connecting the outermost locality records and 



