32 ORGANIC DEPENDENCE AND DISEASE 



to advance without restraint into higher but more transi- 

 tory organisms. 



THE CAMBRIAN FAUNA GENERALLY 



The known Cambrian fauna of North America is repre- 

 sentative of the total life of that age, as its lists are twice 

 the size of all from the rest of the world. The additional 

 species from Europe, Asia, Australia and South America, 

 in which the proportions of immobile and mobile organisms 

 are about as indicated above, make a sum total of approxi- 

 mately 1500 species. In this total the trilobites and other 

 crustaceans constitute 58 per cent; for the North Ameri- 

 can fauna these latter figures are 58.7 per cent. But it is 

 obvious that this fauna was an essentially independent 

 congeries of animals in which we must reckon all the crusta- 

 ceans, all the thin-shelled hyaline pteropods, all the anne- 

 lids, practically all the thin phosphatic and allied brachio- 

 pods (in contrast to their descendents) and perhaps the 

 limpetlike gastropods a fully 90 per cent representation 

 of locomotive freedom. It is an assemblage, too, which, so 

 far as our knowledge extends, was essentially free of ex- 

 pressions of symbiosis, even of the most innocent form. 



PRECAMBRIAN LIFE (ARCHEOZOIC) 



Here lies the field still of greatest importance for future 

 investigations of the beginnings of life. Out of it, thus far, 

 little else than suggestions have been derived as to the ac- 

 tual living things of those vast ages. From the midst of 

 its heaved and altered sediments have been rescued here 

 and there a few tangible fragments of recognizable species 

 of life. From the critical knowledge which is to help most 

 in the unveiling of the progress of life, this difficult reposi- 

 tory is of such high importance that it should enlist the 

 concern of students who are well endowed with patient en- 



