DEVELOPMENT 339 



is generally and probably rightly supposed, then it is an 

 inherited effect which is so strongly impressed on the con- 

 stitution of the fish that preparations for the event begin almost 

 as soon as the fish is hatched. The skull is at first cartilaginous 

 as in any other larva, and there is a curved bar of cartilage 

 above each eye: long before the young fish settles on the 

 bottom, the bar above the eye destined to migrate is absorbed, 

 so that there shall be no obstacle in its path. 



In concluding a chapter on development, it may be convenient 

 to mention very briefly the question of hybrids, that is to say, of 

 individuals that have sprung from an ovum fertilised by a sper- 

 matozoon of an alien species. Such cases are comparatively 

 rare in marine fishes, as far as our knowledge goes, but are not 

 uncommon between the various members of the Salmon family 

 (Salmonidae) , as well as among the members of the Carp tribe 

 (Cyprinidae). The best-known crosses occurring in a state of 

 nature in the former group are Salmon and Trout and Trout 

 and Char, and these are easily made by artificial fertilisation. 

 It has been shown experimentally that the hybrid offspring of 

 Salmon and Trout are deficient in vitality, and seldom — in the 

 case of males never — come to maturity. Dr. Day remarks, 

 concerning the handsome Zebra hybrid of the Trout {Salmo 

 trutta) and the American Brook Trout {Salvelinus fontinalis) that 

 the developing eggs show a very high mortality, and that the 

 resulting offspring are frequently deformed in one way or 

 another. In the Cyprinidae hybrids have been described between 

 Bleak and Chub, Bleak and Dace, Bleak and Roach, Bleak 

 and Rudd, Bleak and White Bream, Bream and Roach, Bream 

 and Rudd, Carp and Crucian Carp, Roach and Rudd, White 

 Bream and Roach, and White Bream and Rudd in the British 

 Isles alone. In these cases the resulting offspring seem to be 

 quite healthy, and, as in the case of the Salmon and Trout, 

 generally exhibit more or less equally the characters of both 

 parent species. In other fishes the characters of the hybrids 

 may be very variable, sometimes resembling one parent, some- 

 times the other. 



