PACIFIC FRESH WATER FISHES. 253 



or stream condition. The most that can be said is that 

 the more fully plated specimens have been taken in small 

 coastwise streams, while the less fully plated ones usually, 

 though not always, have been found farther inland. In 

 general, there is no doubt that the forms having the arm- 

 ature more fully developed are nearer the primitive type. 

 These live near the sea. These facts point to the descent 

 of the species from Gasterosteus cataphr actus. 



The marine species, Gasterosteus cata^phr actus, itself 

 shows little variation. It differs from the well plated 

 form of microcefhalus in having a more slender and com- 

 pressed body, longer spines, longer pectorals, larger pre- 

 pectoral area, more depressed caudal peduncle, and nar- 

 rower pubic plate and thoracic area. These, again, are 

 all differences of degree, and they point to the recent 

 origin of microcephalics and williamsoni, brackish and 

 fresh water forms, derived from those strictly marine. 

 In like manner, doubtless, the partly naked fresh water 

 forms of Eastern America and Europe have sprung from 

 the marine species of the same region. 



