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.” 1 
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 (HO) almost 50 per cent. The 
Mollusca 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. 
