ZOOGEOGRAPHY 299 



Dififerences in Proximity of Apparently Related Forms. — A conspicu- 

 ous zoogeographical fact is that forms of the same gi'oup which are nearly 

 alike are generally near together geographically. An example is found in 

 the Sauritus group of garter snakes, several forms commonly called ribbon 

 snakes. The ranges of these nearly related forms, proximus (Fig. 207), 

 sackeni (Fig. 208), and sauritus (Fig. 209) are geographically near one 

 another, that is, they are not separated by other forms. This proximity 

 of the nearly related forms is diagrammatically expressed in Fig. 210. 

 On the assumption that the relationship is a real one it has been stated 



sauritus 



sackeni 



proximus 



Fig. 210. — Affinities in the Sauritus group of gartersnakes. It is assumed that proxl- 

 tnus is the stem form and has given rise to sackeni and sauritus. The direction of evolution 

 is proximus, sackeni, sauritus or proximus, sauritus, sackeni; or proximus has given rise 

 directly to both sauritus and sackeni. Note that if the course of evolution is jyroximus, 

 sauritus, sackeni, it is concluded that the upper two lines indicate the direction of modifica- 

 tion, that is the range of sackeni lies between those of proximus and sauritus only if the 

 order of origin of the group is proxim.us, sackem, sauritus. (After Ruthven.) 



that directly related forms are generally found in adjoining regions, and 

 not in the same or distant ones. It is doubtful if this statement is always 

 accurate, since the term region may be variously interpreted. It is safe 

 to say, however, that while quite different forms of the same group may 

 occur in the same or neighboring places, a form is geographically near 

 those to which it is immediately related and more distant from those more 

 distantly related to it. 



FACTORS OF DISTRIBUTION 



The factors which determine the distribution of animals in nature may 

 be inferred from the facts of distribution and the attributes of animals. 

 A fundamental fact of zoologj' which may now be considered estab- 

 lished is that each kind of animal has been derived from a preexisting 

 kind. In other words, the various groups are genetically related. Before 

 this was established zoogeography concerned itself largely with the 

 delineation of ranges. This is important work even now, but the ulti- 

 mate goal at the present time is to determine the causes of distribution 

 in general, the factors of distribution in specific cases, and the steps by 

 which the present arrangement over the earth has been brought about. 



