402 Causes and Course of Organic Evolution 



to a primitive cyclostome form, as traced in the next chapter. 

 It becomes then a question of some importance to ascertain 

 whether any or considerable evidence exists for viewing the 

 Cyclostomata as primitively marine or fresh-water. Of the 

 above 5 genera of the group, the species of lamprey or Petro- 

 myzon are now most abundant. Several species of these are 

 purely fresh-water, all of them whether living a part of their 

 life in the sea or at the mouth of estuaries run up rivers to 

 spawn. As now knoTNTi the larval lamprey or Ammocoetes 

 spends several years of its life in fresh water, even though the 

 adult may be frequent in brackish or marine waters. 



Now in most features of organization Peiromyzon is the 

 most primitive genus of the group, and we are amply justified 

 in concluding that the Cyclostomata may as well be claimed 

 for fresh-water as for marine environment. But further the 

 semi-parasitic habit shown in adult life indicates that Myxine 

 and the other genera which are now wholly marine derive 

 their environal relation and often parasitic habit from having 

 been carried to sea by fish hosts like the salmon that are mi- 

 grant between fresh and salt water. Therefore of the living 

 species of Petromyzon we would claim all as fresh-water; the 

 species of the four remaining genera may now be ranked as 

 marine. 



Next to the molluscs the true fishes- of the jjresent day, that 

 include 12,000 species at least, present a serious puzzle to the 

 geographer. When we find 700 or more species that are often 

 modified for abysmal depths of the ocean, almost as many 

 that are inhabitants of high land regions, others that are now 

 land-locked fresh-water forms, and some that carr live for 

 months in a dried up dormant or hibernating state, a slight 

 idea is obtained of the difficulties that surround the problem 

 of determining as to their probable environal origin and generic 

 distribution. 



But tlie fact that at least 90 per cent, of the entire group 

 have become preeminently adapted for aquatic existence, 

 and thus that marine facilities are enormously greater than 

 river, lake, or swamp facilities, must be kept in view. It is 



