58 The Natural History of the Salmon. 



attain to maturity at the same age and at the same 

 size, and as mature individuals generally have the 

 caudal less deeply excised than the immature ones of 

 the same age and size, it is evident that the variations 

 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 inhabit- 

 ing rapid streams as well as still waters show con- 

 siderable 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. 

 Moreover, one and the same individual may pass a 

 part of its life in a lake, and enter a river at cer- 

 tain periods, thus changing the form of its fins 

 periodically. 



" Finally, to complete our enumeration of these 

 variable characters, we must mention that in old 

 males, during and after the spawning season, the 

 skin on the back becomes thickened and spongy, 

 so that the scales are quite invisible, being embedded 

 in the skin." 



We now pass on to another and most interesting 

 subject, the hybridism and distinction, etc. of 



