226 THE SENSE-ORGANS AND PERCEPTION 01^ FISHES. 



tive only of their habits under these artificial conditions. Though 

 the majority of the fishes observed by me^ being inhabitants of water 

 of moderate depth^ may be assumed to be under approximately 

 natural conditions, it is important to bear this reservation in mind 

 in considering the case of conger and other fishes which live in 

 deeper water, and are exposed in the sea to very different influences, 

 especially as regards pressure and light. As an instance of the 

 need for caution in estimating the powers of fishes by their beha- 

 viour in tanks, it may be mentioned that the whiting, though a 

 diurnal feeder, and apparently unable to find its food otherwise than 

 by sight, is nevertheless under exceptional cii'cumstances caught in 

 the sea with a bait on dark nights at a considerable depth. 



It will perhaps be convenient to give a general account of the 

 sense-organs of the animals before describing their habits and modes 

 of perception. 



The Structure op the Sense-organs. 



In examining the sense-organs of fishes I have been a good deal 

 struck with a general fact concerning them, which, though sufl[i- 

 ciently well known and obvious when stated, does not appear to be 

 a matter of a priori necessity, and it may be well to call attention 

 to it in this place. On comparing individual fishes of the same 

 species but of different sizes, it is apparent that the size of the eyes 

 varies with the size of the body. The same fact is true of the 

 scales covering the body, which seem not to increase in number, but 

 in size as the animal grows. In fact, so constant is the number of the 

 scales that, as is well known, they have value as characters for the 

 purposes of classification. In the case of the olfactory organs, in- 

 crease in size takes place both by growth of the individual folds 

 bearing the epithelium, and by the addition of new folds. Now the 

 relation of such an organ as the eye to the organism may be com- 

 pared to that of an instrument to a workman ; and if this comparison 

 is a true one, it is not a little remarkable that the sizes of the two 

 should vary together. The question next arises, is a large eye, 

 cseteris paribus, more efficient than a small one ? It may easily be 

 believed that a larger olfactory organ is more efficient, but such a 

 case as that of the eye seems more difficult. If this question should 

 be answered in the negative, it would be interesting to see if these 

 facts are in hai'mouy with the principle of economy of growth, in 

 obedience to which it is believed that all superfluous parts tend to 

 be eliminated from the body. If it is held by any to be manifest 

 that a larger organ is obviously more efficient by reason of its greater 



