﻿Adaptive Radiation as illustrated by the Genera 

 Creagrutus and Bryconamericus 



Ingress to the portion of Colombia nncler consideration has been 

 difficult. The Cordilleras have long been an effective barrier to 

 the immigration from the Amazon and the Orinoco where tishes are 

 fonnd in great variety. Under such conditions it is not surprising 

 to find that representatives of only one or two branches of a large 

 ^enus have succeeded in gaining entrance, nor that once some mem- 

 ber has gained entrance it should have adapted itself to the great 

 variety of environmental units to be found here. The genus Cre- 

 agrutus gained entrance once and then spread and became modified 

 in various ways. It is encountered by the thousands of individuals. 



To attest to the single origin of all the forms of Creagrutus in 

 Western Colombia we have the peculiar modification in the arrange- 

 ment of the teeth common to all of the species. The radiation which 

 has taken place is given below. 



Ta\ o branches of the genus Bryconamericus have gained access to 

 Colombia. Its representatives are also found in great abundauce. 

 One branch, which at a venture I should say came by way of Lake 

 Maracaibo, has developed a peculiar modification in the caudal 

 and anal of the male. This branch (Argopleura) has given rise to 

 several species. The other branch possibly came from the south. 

 Periianus is found in Peru from the coast to the Beni and its near- 

 est relative is found in northwestern x\rgentina.. It is, however, 

 possible that it has relatives in the unknown regions to the east of 

 the Magdalena basin. 



CREAGRUTUS Giinther. 



Key to the Species of Creagrutus. 



a. Premaxillary with an inner seTies of four or five teeth ; five teeth 



alternating form an outer series of tliree teeth and a middle series 

 of two teeth. 



b. Anal 11 ; eye 2.6 in the head ; suborbitals leaving a narrow naked 



margin ; a silvery lateral band and conspicuous curved dark bar 

 behind the shoulder. Scales about 6-36-2.5 ; depth 4.33. 



1. melanzonus Eigenmann. 



(6) 



