154 Profs. T. W. Bridge and A. C. Haddon. [June 16, 



demonstration of the first would almost necessarily involve the truth 

 of the second, but positive proof of either is extremely difficult. To 

 prove the correctness of the first, it must be shown that there are 

 certain conditions involved in a fresh-water habitat which render 

 the possession of a Weberian mechanism of special value to fresh- 

 water Fishes. If, as we have suggested, the mechanism confers upon 

 all Fishes which possess it an exceptional capacity for locomotion in 

 all directions, with a minimum of muscular effort, it seems reasonable 

 to anticipate that such advantages are of greater importance to fresh- 

 water species than to their marine congeners. 



The poverty of fresh- water faunas, as compared with marine, and 

 the entire absence of certain groups of organisms which are abundant 

 in the sea, are among the most obvious facts in the geographical dis- 

 tribution of animals. Of the special external conditions which have 

 combined to produce these results, the most obvious, and perhaps the 

 most important, are, (a) existence in a medium which, omitting 

 lakes, is always in motion in a definite direction ; and (6) a more 

 precarious and fluctuating food supply, due to climatic severity, 

 alternations of seasons (such as winter and summer, dry and rainy 

 seasons), and to the isolation and comparative smallness of fresh-water 

 areas. On the other hand, the relation of Fishes to other forms of 

 fresh-water life is in many respects unique. No other groups of 

 equivalent taxonomic value approach Fishes in richness of specific 

 differentiation or individual size, and it is equally clear that the pre- 

 dominancy of Fishes is mainly due to the numerical strength of the 

 Ostariophysea3, both in individuals and in genera and species. It is, 

 therefore, not unreasonable to infer that, for some reason, the special 

 conditions of a fresh- water existence are less adverse to Fishes than 

 to any group of organisms, and, further, that the Ostariophyseaa 

 apparently possess an altogether exceptional capacity for adapting 

 themselves to conditions which, in almost all other cases, are inimical, 

 both to individual and specific development. Can it be shown that 

 there is any definite relation between any of the conditions of a fresh- 

 water life and the advantages derivable from the possession of a 

 Weberian mechanism ? 



Of the special conditions of a fresh-water habitat, we lay most 

 stress on the precarious and fluctuating character of the food supply, 

 and a comparison of the relative positions of fresh- water and marine 

 Fishes from this point of view suggests a possible answer to the 

 question. The food supply of marine Fishes is less precarious and 

 less liable to quantitative fluctuations than in the case of fresh-water 

 species, and, moreover, local scarcity of food may be met by migration 

 to areas where, for the time being, food is more abundant. With 

 fresh-water Fishes the converse holds good, and in their case the 

 restricted extent and comparative isolation of fresh- water areas are, 



