110 MEMOIRS OF THE CARNEGIE MUSEUM 



the length in the young M. pacu, as compared with 1.6 in the adult; 2.4 in the 

 length in the young Myloplus rhomboidalis as compared with 1.5 in the adult. 

 Similar changes in shape take place in Serrasalmo rhombeus and probably in all of 

 the other characins with an extremely deep body In this connection figures 2 and 

 3 of Plate LII should also be examined; they represent the change in shape in 

 another type of characins. 



On the Utilization of Different Structures to Obtain the Same End. 



(Plates V-IX.) 



In a number of Siluridce, especially the marine Ariince, the nuchal region is 

 heavily armored. Granting that this armor is an adaptation, it is of interest that 

 it is formed of either the occipital process, the so-called dorsal plate, or combinations 

 of the two in varying proportions. 



In Sciadeichthys flavescens (Plate V, fig. 1) the entire armature is furnished 

 by the dorsal plate. In proops (Plate V, fig. 2) the occipital process takes a slight 

 part, the major portion of the armature being formed by the dorsal p'ate. In 

 parkeri (Plate V, fig. 3) the occipital process is still more prominent. In Sciadeichthys 

 emphysetus (Plate IX, fig. 1) the dorsal plate and occipital process are of about equal 

 value. In the two species of Selenaspis, S. herzbergii and S. passany (Plate VII) the 

 occipital process is slightly larger than the dorsal plate. In Arius spixii and Hex- 

 anematichthys rugispinis (Plate IX) the dorsal plate forms a negligible portion 

 of the armature. It is still further reduced in the three species of Notarius (Plate 

 VIII) in which the occipital process differs progressively from the linear form in 

 N. stricticassis to the leaf-shaped in N. grandicassis. In the latter three species, 

 there is very great variation. 



The series outlined above is not a genetic series. It is simply a series showing 

 the inversely proportioned coordinated development of the structures to accomplish 

 the arming of the nuchal region. They illustrate the dictum of Weismann that 

 "adaptations arise if they are at all possible" and that the structures utilized in 

 bringing out the adaptation are not material. Natural selection would be satisfied 

 with the arming of the nuchal region regardless of the structure that did the arming. 64 



A similar case is furnished by the genera Charax, Asiphonichthys, Acanthocha- 

 rax and Heterocharax. In these a spinous armor for the lower gill-region, or breast, 

 may be considered an adaptation. In the first two genera the clavicle furnishes 

 the spine. Its lower edge rises blade-like and ends in a spine anteriorly and pos- 



64 Dr. C. R. Eastman calls my attention to the fact that a similar armoring has developed in Coccosteus, 

 Dinichthys, and other extinct fishes. 



