THE SKNSE-ORGANS AND PERCEPTION OF FISHES. 227 



size^ it may be asked to what limit it is proposed to carry this 

 principle. Is it applicable to all organs and parts of organs ? Are 

 the cells, for instance, of the tissues of a large individual larger than 

 the similar cells in a smaller one, or would they be more efficient if 

 they were ? It would in any case be most desirable to know in 

 what animals this relation of size between the whole and the parts 

 is found, and to what organs it extends. In particular, it would 

 be of the highest interest to know whether the eggs of a large 

 individual are larger or more numerous than those of a smaller 

 individual of the same species, and so on. An insufficient experience 

 of Crustacea, fishes, and plants leads one to think that in these cases 

 the number of eggs or seeds increases without change of size, though 

 in the absence of more data it is unprofitable to discuss the matter. 

 But as the relation between the size of the body and that of its 

 organs has a high importance in any attempt to obtain a view of 

 the modes of occurrence of variations, these facts in the structure of 

 fishes are emphasized here, in the hope that persons who have the 

 opportunity of handling large numbers of animals may be led to 

 record their observations of similar particulars. 



Eyes. 



The general structure of the eyes of fishes is well known, but 

 some points which have been observed in the course of this in- 

 vestigation may be worthy of notice. Particular attention was paid 

 to the eyes of those fishes which are active at night, in order to see 

 if thei^e is any general common feature among them. The state- 

 ment, for example, is made by Day (British Fishes, vol. i, p. xxviii) 

 that " nocturnal fishes require larger eyes than diurnal ones.^' This 

 may possibly be true, but it is by no means the fact that they are 

 as a rule endowed with larger eyes, as is suggested in the passage 

 quoted. The most typically nocturnal fishes are the conger and the 

 eel. Of these the conger has a large eye, but not a remarkably large 

 one when compared with that of the cod or the bream, which are 

 diurnal, while the eel has a somewhat small eye. The eyes also of 

 the sole, which is emphatically a nocturnal animal, are singularly 

 small, while those of the angel-fish and torpedo are still smaller in 

 proportion to the bulk. Among the rocklings, also night-feeders, 

 the three-bearded rockling has an eye of average size, while that of 

 the five-bearded rockling is decidedly small in proportion to its 

 body. Under these circumstances any general statement is mis- 

 leading. 



Not even is there any uniformity with regard to the presence or 



