218 FRESHWATER FAUNA 



here and there marine animals, which have wandered 

 into them directly from the sea. 



Thus the origin of our freshwater faunas is complex, 

 having been accomplished, sometimes in one way, some- 

 times in another. 



It would appear that the particular forms which 

 succeed in completely adapting themselves to a fresh- 

 water life are relatively stable organisms. Like the marine 

 Lingula * which has persisted from the Cambrian times 

 down to the present day they are capable of much en- 

 durance; they have withstood change of medium, they can 

 suffer extremes of climate, often they survive prolonged 

 desiccation ; and with this hardiness of nature there is 

 possibly associated a rigidity of structure, an absence of 

 plasticity, which has led to their retention of ancestral 

 characters through a long period of time. Such modifi- 

 cations as they have undergone are chiefly in adaptation 

 to the inorganic environment : in the sea, adaptation to 

 the living environment plays a relatively much greater 

 part. 



* Morse, speaking of Lingula and the group of Brachiopods to 

 which it belongs, says : " The tenacity of life manifested by this 

 group is almost beyond belief. The Testicardine Brachiopods do not 

 possess this tenacity, and we may correlate with this fact the infinite 

 diversity of form shown by them since they first made then: appear- 

 ance in geological time." 



