6 Evolution and Distribution of Fishes 



and 902 are marine, this by no means represents the prim- 

 itive distributional relation. 



"We would therefore consider that all palaeontological, 

 structural, and geographical evidence strongly favors the 

 view that the primitive fishes have had a freshwater origin, 

 and that derivative marine forms have evolved from these, 

 which in the case of the anciently derived selacheans, and 

 of the recent teleosteans, reached each to a high climax of 

 species diversity, the former during the Cretaceous period, 

 the latter in our own day." 



If such conclusions are in harmony with the phylogeny 

 and gradual distribution of fishes over freshwater and salt- 

 water areas it should be highly probable that its correctness 

 can be demonstrated by an appeal to the palaeontological 

 record. Further, if that record is sufficiently complete, it 

 should enable us to determine with fair accuracy the ap- 

 proximate period when the main groups and even the 

 smaller families of fishes migrated from a freshwater to 

 a marine environment or vice versa. As a result an evolu- 

 tionary sequence and exactness could thereby be obtained 

 that would be in helpful contrast to the vague and un- 

 certain record of the past. 



In now selecting the group of fishes for detailed treat- 

 ment the writer has been guided by several considerations. 

 First, it has been generally accepted as the group that con- 

 nects the invertebrates with the more evolved vertebrates. 

 Second, it shows several very distinct living divisions that 

 can nevertheless be traced back through the greater extent 

 of the palaeontological record to forms that strongly 

 suggest community of origin and structure ; Third, from 

 its environal relations, and so from causes that will be 

 fully discussed later, the group is one that is surprisingly 

 well represented from later Silurian times up to the present 

 day. Fourth, in spite of the great amount relatively of 

 soft tissue that makes up each individual, the early for- 

 mation, from at least Silurian times onward, of external 

 scales and later of a resisting bony framework, ensured 

 preservation of the remains in an exceptionally favorable 

 manner. Fifth, owing to rare but remarkable combina- 

 tions of geological conditions, that are fully discussed later 



