108 



CALIFORNIA FISH AND GAME 



All trout of this species found elsewhere in the Lahontan Basin 

 are similarly spotted. An enumeration of the spots of several speci- 

 mens follows : 



Fig. 21. Llewellyn Fall.s, the barrier 

 which prevents the passage of trout of 

 the species Salmo hensliaivi into the 



basin 

 Photo 



occupied 

 by E. L. 



by Salmo 

 Macaulay. 



It is a well-known 

 isolated animals tend to 



seleniris. 



biological 



These fish measured from 14^ 

 to 28 J inches in length. The tspots 

 of «S'. henshau'i are so numerous, 

 large and conspicuous that the 

 .species is commonly referred to by 

 liatcheiy men as the black-spotted 

 trout. 



Recognizing that the trout 

 above Llewellyn Palls may be 

 regarded as members of a species 

 which is different from that below, 

 one asks concerning their origin 

 and the means by which they 

 arrived. The immediate and only 

 answer is that they have been there 

 as long as any trout have been in 

 the Carson Basin. The falls are 

 of more recent origin than the 

 trout. Before the falls became a 

 barrier to free migration, the trout 

 of the streams of Fish Valley were 

 not different from others in the 

 river below. When the falls became 

 an obstruction to the passage of 

 fish those above became insular. 

 Some of their number might 

 occasionally drift down over the 

 falls to be ultimately lost in a 

 large poyiulation, but none from 

 below could ever mingle Avith those 

 above. 



principle or law that insular or 

 change from the parent stock. Just 



vary or 

 why they may vary in any particular way is a matter for speculation 

 It is not possible at present and in this jiarticular case to account for 

 the unusual brilliancy of color, or for a loss of the black body spots. 

 Whatever else may be said, the significant thing is that we have 

 here an exceptionally good illustration of variation through isolation. 

 The habitat of the species is sharply defined, the barrier which pro- 

 tects it is perfectly evident, and the parent stock from which the 



