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



their food supply, economy in the expenditure of muscular energy 

 must be of primary importance to the majority of fresh- water Fishes, 

 and more particularly to those which, wholly or in part, derive their 

 food from vegetable sources. Hence, the possession of any mechan- 

 ism which will not only remove all restrictions to motion in the 

 vertical direction, and thereby enlarge the area within which food 

 may be obtained, but, at the same time, will also enable those Fishes 

 fco execute all locomotor movements with the least possible expendi- 

 ture of muscular effort, must prove to be a great physiological 

 advantage to them, inasmuch as economy of muscular effort implies 

 diminished tissue metabolism, and, consequently, indirectly but effec- 

 tually aids the accumulation of the nutritive reserve, at the expense 

 of which the Fish subsists during the lengthened periods when fresh 

 food is relatively scarce. It may be that this is precisely the advan- 

 tage which the possession of a Weberian mechanism enables all 

 Ostariophysese to realise an advantage which, as we venture to 

 suggest, is one of the main causes of their marked ascendency over 

 all other fresh- water species in which this mechanism is wanting, and 

 with which they come into direct competition. 



As no other attempt has been made to associate the evolution of 

 the Weberian mechanism with any special peculiarities of external 

 environment, we would suggest the following tentative conclu- 

 sions : 



1. The special feature of a fresh-water habitat that has conditioned 

 the development of the Weberian mechanism in the Ostariophysese is 

 the occurrence of seasonal or periodic quantitative variations in the 

 food supply, variations to which the Ostariophyseae, from their 

 herbivorous or omnivorous habits, are specially liable. 



2. In view of such unfavourable nutritive conditions, the special 

 advantage which is conferred upon the Ostariophysese by the posses- 

 sion of the Weberian mechanism is a capacity for executing locomotor 

 movements in any plane, with an almost irreducible minimum of 

 muscular effort and tissue metabolism. 



3. If a variable and inconstant food supply is to be regarded as one 

 of the inevitable conditions of a fresh-water existence, and necessitates 

 strict economy in the expenditure of muscular energy, any mechanism 

 which secures this result must be of unquestionable importance to 

 the species, and hence it may be that the Ostariophysess owe their 

 dominant position among fresh-water Fishes to the possession of the 

 Weberian mechanism. 



4. The evolution of the Weberian mechanism has not only condi- 

 tioned the predominancy of the Ostariophyseae, but, indirectly, has 

 favoured the existence in fresh water of a large number of purely 

 carnivorous Fishes, which depend on the former for their food, and 

 therefore may also be regarded as one of the primary causes of the 



