24 The American Naturalist. [January, 
any locality, when free intermigration is possible, seems strange. 
An analgous condition is to be found on the Galapagoes 
Islands. Dr. Baur tells me that islands within plain sight 
of each other harbor distinct varieties of the same species of 
birds which could readily intermigrate, but do not. 
This raises the question of the sort of influence exerted by 
the environment. Is it merely selective, or is it directive? Is 
the variation promiscuous and inherent in the species, or is it 
determinate and forced in certain directions by the environ- 
ment? The latter seems to me the better way of reading such 
conditions as are represented by the many curves which show 
a greater variation towards an increased number of rays than 
towards a decrease of rays. Here the variation is not promis- 
cuous, but definitely determinate. See, in this connection, the 
curve for all the specimens. 
The origin of new varieties is admirably illustrated by the 
curves for Lake Washington and Umatilla. In these, two dis- 
tinct peaks are found. While no varietal value is claimed for 
these peaks, isolation of members of such peaks, either physio- 
logically or locally, would tend to establish such incipient 
varieties. 
EXPLANATION OF FIGURES. 
The vertical lines, in all cases, stand for a definite number 
of anal rays. The total height of the figures represent 100 per 
cent., and the height of the curves, at any point, the per cent. 
of specimens having the particular number of rays in the anal. 
Fig. 1. Curve of variation for 217 specimens of Leuciscus hydro- 
phlox from the upper Snake, and for 825 specimens of 
Leuciscus balteatus from many localities, varying from 1 
to over 5000 feet in elevation. 
a, the two series of specimens are combined in the 
broken line curve. 
Fig. 2. Three curves showing the variation of the three locali- 
ties represented from the Frazer system : 
Griffin Lake, 1900 feet, 17 specimens. 
Sicamous, 1300 feet, 58 specimens. 
Mission, 1 foot, 79 specimens. 
