10 The American Naturalist. [January, 
LEUCISCUS BALTEATUS (RICHARDSON), A STUDY 
IN VARIATION: 
By Cari H. EIGENMANN. 
Nowhere else in North America do we find, within a limited 
region, such extensive variations among freshwater fishes as on 
the Pacific slope. This is true, whether we have reference to 
the extent of variation between the extremes of the same family 
or to the limits of variation in any given species. 
A comparison of the members of the eight families of fishes 
having representatives on both the Atlantic and Pacific slopes, 
show that, on an average, each of these families has four 
genera and sixteen species on the Pacific slope, and seven 
genera and thirty-six species on the Atlantic. Yet, although 
the number of species is more than twice as great on the At- 
lantic slope, the variation in the number of fin rays among 
the Pacific slope species is greater in all but two families. I 
have recently” made a detailed comparison between the mem- 
bers of the different families, and there attributed this great 
extent of variation to two causes. First: the fauna is of 
diverse origin; some of the members are of Asiatic, while 
others are of Atlantic descent. Second: the fauna is new as. 
compared with the Atlantic slope fauna, and has not yet 
reached a stage of stable equilibrium. It is possible, as sug- 
gested to me by President Jordan, that the Pacific slope fauna 
has retained its primitive charactérs more nearly than the 
Atlantic slope fauna, which shows signs of degeneration in its 
fins and teeth. 
This great variation between the members of the same fam- 
ilies is not confined to the fin rays. It is equally true of other 
characters, but can best be demonstrated in characters whose 
variation can be numerically expressed. The pharyngeal 
1 Contributions from the Zoological Laboratory of the Indiana University, No. 
11. 
? Results of Explorations in Western Canada and the Northwestern ‘United : 
States. Bull. U. S. Fish Com. for 1894, pp. 101 to 132. Plates 5 to 8. June, 1894. 
