lxxxii DIFFUSION OF FISHES. 



they would have left any trace unless it were their horny teeth. Indeed 

 specimens of such teeth, or those of molluscs or annelids have been discovered 

 in the Lower Silurian and Devonian strata. 



Considerable difficulty has arisen as to whether the ancestral forms* of 

 fish were fresh-water or marine, and if the latter, whether they were first 

 littoral, subsequently pelagic, or originally pelagic. At the present time, 

 investigations tend to show that although marine species not infrequently 

 enter, breed, and even take up their permanent abode in fresh waters, such 

 is not the case with strictly fresh-water forms, which do not voluntarily 

 enter the sea, or if there would succumb. True, a flood from a river or 

 inundation along a coast may temporarily so alter the composition of the 

 sea- water that a fresh-water form may reside in it or travel by it for some 

 distance along a coast-line, but they die when the water becomes normally 

 saline. 



M. Agassiz, when giving some interesting observations on this question, 

 remarked that although at present it is not plain that fluviatile types on the 

 whole are superior to the marine ones, still among the higher forms of 

 Chondropterygii as in ganoids, the bony pike, Lepidosteus, of America, the 

 Polypterus of tropical Africa^ the Lepidosiren or Protopterus of the west 

 coast of Africa, and the Geratodus of Queensland, are restricted to fresh 

 waters. While even in groups considered to be marine the fresh-water forms 

 possess characters generally denoting their superiority over their marine 

 representatives. 



DIFFUSION OF FISHES. 



Fishes may have been dispersed over the surface of the globe in several 

 ways, as marine ones through the ocean, and fresh-water species along rivers 

 and watersheds, and in exceptional instances along coasts. For similarly to 

 other forms of life they possess a natural tendency to increase as well as 

 disperse when not checked by climatic or physical difficulties, or other 

 external causes. As assisting in this diffusion there are uncertain agencies 

 which can only act occasionally, but by means of which fresh-water forms 

 may be dispersed, such as floods carrying them from one watershed into 

 another, which although contiguous at their sources may lead in entirely 

 different directions. In some species it has been shown that if fecundated 

 eggs, as of trout, are kept moist and cold they can be conveyed in safety for 

 long distances (page lxvii), while salmonoid eggs removed from the maw of 

 a trout have been hatched. This renders it not improbable that should a 



* In embryo-fish the developmental changes when correctly ascertained are a guide to the 

 classification of the species. 



