316 



PRINCIPLES OF ANIMAL BIOLOGY 



sauritus 



sacken 



proximus 



sauritus 



Proximity of Related Forms. — If species originate from other species 

 it would be expected that a very young species would still be near its 

 progenitor. It would not have had time to travel very far. If the 

 youth of species and the sources from which they have sprung be judged 

 from the similarity between species, this expectation is in general realized. 

 Those species of a genus, or those subspecies, which are most nearly 

 alike are found geographically near one another. An example is found 

 in a group of garter snakes known as ribbon snakes. The forms in 



question are three subspecies of one 

 species, Thamnophis sauritus. One sub- 

 species, called proximus, occupies a range 

 west of the Mississippi River and along 

 the east coast of Mexico (Fig. 270); 

 another, named sackeni, is in Florida and 

 on the Gulf Coast east of the Mississippi ; 

 the third, sauritus, is east of the Mississippi 

 and north of Florida. The three ranges 

 are practically in contact wdth one 

 another; at any rate they are not sepa- 

 rated by ranges of other garter snakes. 

 Other species of garter snakes are at a 

 distance. 



The earliest of these subspecies, as 

 judged from their characteristics, appears 

 to be proximus; from it sackeni and 

 sauritus must have sprung. The order 

 in which the latter two forms arose is in 

 doubt. The possibilities are portrayed 

 by Fig. 271; proximus may have given 

 rise to sackeni and sauritus separately, or 

 it may have produced one of them (either 

 one), and this in turn produced the other. 

 The principle that nearly related (that is, similar) animals are geo- 

 graphically near one another is illustrated also in the higher taxonomic 

 categories. The genera of mammals east of the Rocky Mountains in 

 the United States have more similarities among themselves, and the 

 genera of the Pacific coast area more mutual likenesses, than do the 

 eastern genera to the western genera. The principle holds even for 

 continents. The animals of one continent are usually more alike than 

 they^are like those of any other continent. Moreover, the faunas of 

 neighboring continents are more alike than are those of more distant 

 continents. The animals of North America and Eurasia are particularly 

 good examples of this phenomenon. The similarity of these two faunas 



proximus 



sackeni 



sauritus 



sQckeni 



proximus 



Fig. 271. — Three pcssible ori- 

 gins of subspecies sauritus and 

 sackeni from proximus, in a garter- 

 snake species. {After Ruthven.) 



