318 ZOOLOGY. 



with mere individual diiierences is also applied arbitrarily 

 and for mere couvenieuce' sake." (Origin of Species, 

 P- 62.) 



Varieties are groups of individuals breeding true to each 

 other, which resemble each other in color, size, etc., i.e., in 

 characters less pronounced than spiecific ones. Darwin 

 states that a variety is an incipient species, or a species in 

 process of formation. 



Local varieties, says Wallace, are the first steps in the 

 transition from varieties to spiecies. They are usually 

 restricted to small circumscribed areas, sejwrated by moun- 

 tains, or by altitude, or by moist or dry areas; or, if marine, 

 by different kinds of bottom, sucli as sandy, muddy, or 

 rocky, or different degrees of saltness of the water. Common 

 examples are the local varieties of Littorinti littorea, Pur- 

 pura lapilluf:, and the lobster, the latter even within a single 

 bay, as Casco Bay, Maine, varying in different situations 

 and different parts of the bay. 



In the fresli-water fishes of the Pacific coast each locality 

 has a variety, wliicli in the aggregate is different from the 

 variety of every other locality (Gilbert and Evermann). 

 These variations are due to the different environments, as 

 the climatic, altitudinal, and geological differences in 

 the different streams, and on tlie Pacific slojae even in the 

 course of the same streams they are very great (Eigen- 

 mann). Species of moths which do not vary much on the 

 Atlantic coast are also very variable in California. 



Even in two neighboring lakes in Indiana (Lakes Tur- 

 key and Tippecanoe) the individuals of a darter {Etlieos- 

 toma capKodes) from one lake differ constantly from those 

 of the other lake in color, in the scales of the nape and of 

 the lateral line, in the number of spines in the anal fin, in 

 the number of dorsal spines and rays (Moenkhaus). Sim- 

 ilar instances are the absence of ventral fins in some of the 

 fishes inhabiting even widely separated mountain lakes, and 

 the presence of enlarged scales along the base of tlie anal 

 fin in the Cypriuidse inhabiting the mountain streams of 



