178 ■ PISHES. 



Zepidosteus. Gastrosteus is truly polygamous, several females 

 depositing their ova into the same nest, guarded by one 

 male only. Some Teleostei (OpMocephalus), and probably all 

 Chondropterygians, are monogamous ; and it is asserted that 

 the connection between the pair is not merely temporary, but 

 lasts until they are separated by accident. Monogamous are 

 probably also all those Teleosteans which bring forth living 

 young, and those, the males of which, for the attraction of 

 the female, are provided with appendages, or ornamented 

 with a bright coloration. 



Hybridism is another source of changes and variations 

 within the limits of a species, and is by no means so scarce 

 as has been believed hitherto ; it is only apparently of ex- 

 ceptional occurrence, because the life of fishes is more with- 

 drawn from our direct observation than that of terrestrial 

 animals. It has been observed among species of Serranus, 

 Pleuronectidce, Gyprinidce, Clupeidm, and especially Salmonidce. 

 As in other animals, the more certain kinds of fishes are 

 brought under domestication, the more readily do they inter- 

 breed with other allied species. It is characteristic of hybrids 

 that their characters are very variable, the degrees of affinity 

 to one or the other of the parents being inconstant; and 

 as these hybrids are known readily to breed with either of 

 the parent race, the variations of form, structure, and colour 

 are infinite. Of internal organs the dentition, gill-rakers, 

 pyloric appendages, are those particularly affected by such 

 mixture of species. 



Some fishes are known to grow rapidly (in the course of 

 from one to three years) and regularly to a certain size, growth 

 being definitely arrested after the standard has been attained. 

 Such fishes may be called " full-grown," in the sense in which 

 the term is applied to warm-blooded Vertebrates — the Stickle- 

 backs, most Cyprinodonts, and many Clupeoids (Herring, 



