266 Evolution and Distribution of Fishes 



Temperate zone. In such environment, the true Sticklebacks 

 construct special nests for the eggs and young. But they 

 have invaded, and become thoroughly at home in, brakish 

 water, while several species, such as the common G. aculea- 

 tus inhabits equally American and European seashores. 

 The more evolved and abundantly spined Pygosteus is es- 

 sentially marine, while Spinacia vulgaris is the Sea Stickle- 

 back. 



That we have here to do with a real case of seaward 

 migration of a primitively freshwater group is perfectly 

 indicated by some observations of Jordan who says {2=,^:- 

 229) : "The sticklebacks inhabit brakish and freshwaters 

 of Northern Europe, Asia and America. The same species 

 is subject to great variations. The degree of development 

 of spines and bony plates is greatest in individuals living 

 in the sea, and in clear streams of the interior. Each 

 of the mailed species has its series of half-mailed or even 

 naked varieties found in the freshwaters. This is true in 

 Europe, New England, California and Japan. The further 

 the individuals are from the sea, the less perfect is their 

 armature. Thus Gastrosteiis cataphractus, which in the 

 sea has a full armature of bony plates on the side, about 

 30 in number, will have in river-mouths from 6 to 20 plates, 

 and in strictly freshwater only 2 or 3 or even none at all." 



Here we would claim is a clear case where migration 

 from fresh to salt water has still left us in the individuals, 

 the varieties, the species and the genera of a group, with 

 morphological as well as distributional stages of progres- 

 sion that become highly indicative for other but less plastic 

 genera or families. 



The Aulostomidae and Fistulariidae are now two di- 

 vergent but related families, that have branched off from 

 some member or near ally of the Gastrosteidae, but which 

 show connection with it by the Tertiary freshwater genus 

 Protosyngnathus of Sumatra. Both families however, have 

 now become marine, as are the still more divergent, but 

 derivative families Centriscidae, Syngnathidae, and Pegasi- 

 dae, some of which have become remarkably modified, as 

 for example the flute-mouths, sea-horses, and pipe-fishes of 

 tropical and semitropical seashores. 



