634 FISHES. 



front part of the head happens to be going on while the 

 individual is able to obtain only a scanty supply of food, the 

 usual proportions of the head and trunk are so altered that 

 the species is very difficult to recognise. Barren male fish 

 approach the females in the proportions of the head' and 

 body, but hybrid fishes do not differ in this respect from their 

 parents. The abundance or scarcity of food, and the disposi- 

 tion or indisposition of the Salmonoids to feed, are other 

 causes affecting the growth or fulness of the various parts of 

 the body. In weU-fed fishes the head is proportionally not 

 only smaller but also shorter, and vice versa. 



The Jins vary to a certain degree. The variation in the 

 number of the rays is inconsiderable and of no value for 

 specific distinction. The caudal fin undergoes considerable 

 changes of form with age, and dependently upon the sexual 

 development. Young specimens of all species have this fin 

 more or less deeply excised, so that the young of a species 

 which has the caudal emarginate throughout life, is distin- 

 guished by a deeper incision of the fin, from the young of a 

 species which has it truncate in the adult state. As the 

 individuals of a species do not aU attain to maturity at the 

 same age and at the same size, and as mature individuals 

 generally have the caudal less deeply excised than immature 

 ones of the same age and size, it is evident that the varia- 

 tions in the form of the caudal are considerable and numerous, 

 and that it is a very misleading character if due regard be not 

 paid to the age and sexual development of the fish. Further, 

 species inhabiting rapid streams as well as still waters show 

 considerable variations in the form and length of all the fins ; 

 those individuals which live in rapid streams, being in almost 

 constant motion and wearing off the delicate extremities of 

 the fins, have the fin-rays comparatively shorter and stouter, 

 and the fins of a more rounded form, particularly at the 

 corners, than individuals inhabiting ponds or lakes. More- 



