227 



The most marked difierence is found in those fishes which hare been 

 separated from their Atlantic slope relatives long enough to become gen- 

 erically distinct. In several genera, 3feda, Lepidomeda, Richardsoniu>< (sub- 

 gen.) Columbia the modifications of the fins mark the genus. 



The more striking modifications are the following : 



In the sub-genus Richardsonim, confined to the Columbia and to the 

 Fraser systems, the number of anal rays varies from 12 to 25, which is an 

 increase of from 2 to 15 rays over the species of Leuciscus, some of which 

 have also (more recently) entered the head waters of the Columbia but 

 the great bulk of whose species inhabit the Atlantic slope. 



The genus of Oncorh i/nchus confined to the Pacific slope has a similar in- 

 crease of anal rays over Sabno and Salvelinus which are genera of wider 

 distribution, some of the species entering streams of the Pacific slope. 



The above are examples of the addition to the number of rays in the 

 fins. 



The modification of some rays into strong spines is seen in the Min- 

 nows, 3feda and Lepidomeda. Among the many Atlantic slope Minnows 

 none have spines in their fins while Meda and Lepidomeda, confined to the 

 Rio Colorado, have two spines to the dorsal fin, the posterior received in- 

 to a longitudinal groove of the anterior. 



The increase of spines is very strikingly illustrated in the Sun Fishes 

 (Centrarchidse) in which the single Pacific slope species, Archoplites inter- 

 ruptus (Girard), has one more dorsal spine than any of the numerous At- 

 lantic slope relatives. 



We must either assume that there has been in all the families a fortui- 

 tous variation in one direction which has enabled natural selection, 

 which favored larger finned fishes to produce the present results, or we 

 must admit that the environment has afi"ected all species alike and the 

 effect of this action has by heredity become gradually the established or- 

 der of things. In other words we must either explain the conditions with 

 the Neodarwinians through natural selection or we must with the NeoLa- 

 marckians believe in the acquisition of larger fins through greater use of 

 these organs and the subsequent transmission of the modification to the 

 offspring. 



These modifications being in one direction are unquestionably due to 

 one definite environmental cause. What that cause is I am unable to 

 say. A comparatively short, swift, water course suggests itself most natu- 

 rally but the species inhabiting the short, swift, streams of the western 



