34 ORGANIC DEPENDENCE AND DISEASE 



true example of the deathless life wherein reproduction by 

 division has carried the parent into all its uncountable prog- 

 eny. 



Once more it is well to enforce the fact that the simplest 

 organisms have lived the longest and those that have so 

 lived have been subjected to the minimum of change and 

 the optimum of adaptation. While we recognize that to 

 this the sessile condition and immobility arising from any 

 other cause contribute, it is such persistent simple forms 

 that Ruedemann has called ''immortal types. "^ 



THE COMPOSITION OF THE LOWER CAMBRIAN 

 FAUNA IN NORTH AMERICA 



This is the "first fauna." The casual remnants of life 

 that have been found in the Precambrian rocks cannot be 

 characterized as fauna or flora. And this "first fauna," 

 so far as known to us, must be regarded as an escape from 

 unfavorable conditions, for its sediments have everywhere 

 been easily liable to alteration by earth movements and 

 destruction of its organic contents. So it is fair to say that 

 much of the fauna is still to be uncovered. In its known 

 composition, however, which is now numerically estimated 

 at 243 species in North America, there is essentially the 

 same relative prominence of groups of organisms as in the 

 total Cambrian; thus the brachiopods (76) constitute about 

 30 per cent, the trilobites (110) alm.ost 50 per cent. The 

 MoUusca are represented chiefly by the gastropods (16 

 species), mostly of the simple, conical, limpet shapes and 

 the free-swimming pteropods (12 species). Otherwise there 

 are representatives of algae (2), sponges (1), corals (8), 

 annelids (trails; soft bodies not retained), cystids (ele- 

 mentary echinoids) (2), pelecypods or clams (1), eucrus- 

 taceans or shrimps (5). That the percentage of locomotive 



1 Op. cit., p. 116. 



