ELASMOBRANCH BRAIN ORGANIZATION 119 



realized if one can imagine a concept of mammalian morphological variation 

 and general biology based only on rats and armadillos. 



Preoccupation with recognition of common morphological features and 

 their phylogenetic levels clearly dominated early comparative neurobiological 

 studies (Papez 1929, Kappers et al. 1936), and such considerations are, 

 regrettably, more evident in contemporary comparative neurobiology 

 (Crosby et al. 1967, Sarnat and Netsky 1974) than in other comparative 

 disciplines. This is perhaps due to the strongly human-oriented nature of 

 much vertebrate neurobiology. 



Neural features common to all vertebrates tell us little about specific 

 adaptations, and thus little about the evolution of any vertebrate group. At 

 best, features common to widely divergent species provide clues to the origin 

 and initial adaptations of vertebrates. It is differences in morphological 

 features that signal adaptation and, thus, evolution. 



It is equally fallacious to characterize neural features of different vertebrate 

 species as points on a linear, simple-to-complex scale with mammalian neural 

 organization at the acme. Vertebrate evolution is not a unilinear hierarchy, 

 but rather a series of radiations, widely divergent and separated since the 

 early Devonian period. Within each of these radiations, adaptations have con- 

 tinued to occur; and frequently similar, as well as different, solutions have 

 evolved in response to complex environmental forces. 



I believe a main focus of comparative neurobiology should be to sample 

 brain variation among living vertebrates and to recognize different morpho- 

 logical patterns and their adaptive significance, rather than reconstructing a 

 unilinear phylogenetic history of vertebrate brains "from fish to man." Only 

 by sampling the existing variation can adaptive patterns be recognized. 

 Vertebrate features are not like the elements of the periodic table— it is 

 impossible to predict what variation should exist based on an incomplete 

 sample, since evolution is opportunistic. Once the sampling is fairly com- 

 plete, patterns can be recognized and hypotheses about the biological 

 significance of such patterns can be formulated and tested. 



For example, several species in two or more vertebrate radiations have 

 independently evolved large brains with separate and complex sensory and 

 motor representations in homologous brain centers. Do these adaptations 

 reflect similar life styles? Perhaps we can recognize certain behavioral and 

 ecological correlates that are always associated with a particular morpho- 

 logical pattern. Perhaps a given pattern has evolved a number of times as 

 the most advantageous pattern (if not the only pattern genetically possible) 

 for any group of vertebrates using the environment in a particular way. 



In this chapter, the gross brain variation in cartilaginous fishes is described, 

 and different patterns of neural organization are recognized. Our present 

 knowledge regarding CNS organization in cartilaginous fishes is reviewed. 

 Neural similarities among different groups of cartilaginous fishes are noted, 

 and comparisons are made with other vertebrate groups. Many of the 

 patterns recognized have most likely evolved independently, and their pos- 

 sible adaptive significance is discussed. 



