1892.] Contributions to the Anatomy of Fishes. 155 



in general, an insuperable obstacle to migration as a means of 

 remedying local scarcity. The poverty of most other forms of fresh- 

 water life absolutely conditions the existence of a relatively larger 

 number of herbivorous or omnivorous Fishes in fresh waters than in 

 the sea, where the abundance and variety of other animal organisms 

 are so much greater, and this necessarily involves the existence of a 

 piscine fauna, which, from the nature of the food supply, is to a 

 large extent peculiarly liable to the exigencies of a precarious and 

 inconstant food supply. It can also be shown that marine Fishes are 

 more voracious than fresh- water, and that while the latter may 

 survive total abstinence from food for weeks or months, the former 

 succumb within a few days. The majority of the Ostariophyseae 

 appear to be herbivorous or omnivorous, while the capacity of many 

 of them for accumulating reserve food material, at the expense of 

 which they live during the seasons of relative scarcity, has often been 

 remarked. 



Not only do marine Fishes differ from the majority of fresh- water 

 species in the greater constancy and abundance of their food supply, 

 but they also differ from the latter in their method of pressure 

 adjustment. Relying, as the former do, upon the relatively slow 

 processes of gaseous secretion and absorption, any departure from the 

 plane in which, for the time being, they are in equilibrium must 

 involve a decrease or increase of specific gravity to an extent propor- 

 tional to the amount of pressure variation. Hence, in ordinary rapid 

 locomotor movements, involving more or less extensive changes of 

 level and pressure, there mast be an increase of muscular exertion, 

 which will necessarily be greater in proportion as the Fish departs 

 from its normal plane of equilibrium. In the great majority of 

 fresh-water Fishes, that is to say, in the Ostariophysese, pressure 

 adjustment is more accurate and rapid, so that in all variations of 

 level and pressure, whether rapidly or slowly brought about, but 

 more especially under conditions of diminished pressure, the Fish 

 always retains a normal plane of least effort, with the result that its 

 locomotor movements will be accompanied by a minimum expenditure 

 of muscular effort. It may therefore be inferred that, as a general 

 rule, marine Fishes are exposed to greater demands upon their 

 muscular energy than is the case with fresh-water species a differ- 

 ence which must always be associated with the more favourable 

 nutritive conditions under which the former exist, as compared with 

 the more precarious food supply of the latter while, at the same 

 time, it affords a reasonable explanation of the relative capacities of 

 marine and fresh-water Fishes for enduring prolonged abstinence 

 from food. 



If we do not overrate the importance of these considerations, it is 

 obvious that, in view of the precarious and fluctuating character of 



