396 Causes and Course of Organic Evolution 



a long interval of time existed between the early cambrian 

 wlien tlie first gastropods appeared, and the silurian when 

 laniellibranchs seem to have originated. AVhether primitive 

 soft species ^dth rudimentary bivalves may then have evolved 

 from a fresh-water ancestry is only a conjecture for which we 

 have no fundamental evidence. This is further discussed 

 on p. 523. 



The Amphineura and Cephalopoda seem to resemble the 

 last in their marine ancestry, and equally in the entire absence 

 of any types that would connect them primitively with a fresh- 

 water origin. 



In reviewmg the molluscs now it might well be emphasized 

 that, considering the abundance of genera of living fresh- 

 water and land gastropods, in view of the undoubted early 

 origin of these in cambrian and almost certainly in the late- 

 archsean period, in view of the extensive changes and denuda- 

 tion action going on then in the land areas, and in view of 

 the wide possible range for life of aquatic forms when they 

 reached the sea, we would suggest strongly a primitive fresh- 

 water origin for the entire group in the late-archa^an, an early 

 separation of the Gastropoda into several fresh-water and land 

 areas of evolution as well as into several marine lines, and a 

 later origin of the now exclusively marine groups of Scaphopoda, 

 Amphineura, and Cephalopoda, as well as of the mixed lamel- 

 libranchs, from jjrimitively fresh-water ancestors of simple 

 structure. 



The Arthropoda include the four solid groups of the Crus- 

 tacea, the Arachnida, the Myriapoda, and the Insecta. The 

 first of these alone deserves to be regarded as marine, the others 

 are in truest sense land or fresh-water organisms. But, when 

 the comparative structure, tjie embryology, and the taxonomy 

 of the Crustacea are examined in detail, one finds that the 

 genera and even species of the acknowledgedly primitive group, 

 the Entomostraca, are nearly all fresh-, rarely brackish-Avater, 

 still more rarely marine. Here again, however, as in previous 

 examples, when members became thorouglily marine, they 

 'ranched out into several phyla and many genera, that show 



