VARIATION 363 



part of the course of the river, and fourteen specimens were 

 taken still higher up at an elevation of 1900 feet; in the first 

 sample the most frequent number of ventral rays was 19, in 

 the second it was 17, in the third it was 15. Thus the number 

 of rays decreased with the altitude and at the same time the 

 range of variation was also decreased. Similar results have 

 been obtained in other species, and are attributed to the great 

 differences in climatic and geological conditions, not only 

 in different streams, but in different parts of the same stream, 

 on the Pacific slope. Of course such local differences could 

 only exist in a non-migratory species such as the Leuciscus in 

 question, but as was pointed out above we do not know how 

 far the differences are permanent and inherited, and whether 

 they would disappear in fishes which developed and grew in 

 the same locality although produced by different local varieties. 

 Professor Eigenmann discusses the question of the kind of in- 

 fluence exerted by the environment, asking whether it is merely 

 selective or directive : " Is the variation promiscuous and in- 

 herent in the species, or is it determinate and forced in 

 certain directions by the environment? The latter seems to 

 me the better way of reading such variations 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 promiscuous but definitely determinate." 

 It is possible in this case that the variation is connected with 

 the increase or decrease in the functional use of the fin, but in 

 other cases no direct influence on the varying organ can be 

 traced. 



Another remarkable case in which a relation of variation to 

 local conditions is evident is that of the trout. A complete 

 series of transitional forms has been traced between the common 

 brook-trout and the anadromous sea-trout. They vary in 

 form and colour, in the number of caecal appendages of the in- 

 testine and in the vomerine teeth, while the differences in adult 

 size are enormous. There are land-locked lake-trout like that 

 of Loch Leven, small brook-trout like those of Cornwall, never 

 more than a few inches in length, and estuarine trout which 

 are scarcely distinguishable from true sea-trout, which in its 

 turn is different on almost every part of the coast. Numbers 

 of species of trout have been named by various systematic 



