Evolution of Animals 403 



not surprising therefore that of the 12,000 to 13,000 species 

 already recorded about 8000 species or more are marine, fully 

 4000 are fresh-water, and a few hundred like the salmon, the 

 blenny, and the stickleback show surprising adaptability to 

 salt-, brackish-, and fresh-water conditions. iVs with the lam- 

 prey so with many of the adaptable forms like the salmon, 

 their present-day instincts and irrito-environal responses cause 

 them to seek the sea or even the ocean as a great and rich 

 feeding ground, then to return to rivers where their inherited 

 instincts cause them not merely to spawn, but to excavate 

 special cavities amid stones or gravel for this purpose. 



The entire question of the relation of the spawning migra- 

 tional or anadromous fishes, to their phylogeny on the one 

 hand and their inherited or acquired habits on the other, is 

 one of intense interest, but is one also that has yet to be thor- 

 oughly investigated in light of all the facts of palseontology, 

 of embryology, of structure, and of taxonomy. That the 

 "homing" instinct is one which may be lost, or more or less 

 retained, or again acquired seems to be indicated in the case 

 of the sea lampreys that return to the rivers, in those fishes 

 which now seem to breed in deep sea recesses, and in the salmon 

 that waver between the brook trout on the one hand and the 

 marine or Quinnat salmon on the other. 



We would shortly sum up our conclusions by saying that 

 the Cyclostomata were probably, as they still are in part, 

 of fresh- watei origin; that the Selachii are and have been 

 through long epochs nearly all evolved as marine forms; that 

 the Polypteridse are and probably have been fresh-water in 

 habit and history; that the Dipneustei are similar to the last; 

 that the Chondrostei or spoonbill and sturgeon series agree 

 with the two last except that some of the sturgeons pass into 

 the sea to feed and return inland to spawn; that the Holostei 

 are now wholly fresh- water; that of the 13 suborders of the 

 Teleostei given by Boulenger the first or IVIalacopterygii are 

 almost wholly fresh-water, the third or Symbranchii are mainly 

 fresh-, more rarely brackish-water inhabitants, the fourth 

 or Apodes — that includes the eels — are rarely fresh-water. 



