AND ITS INHABITANTS 113 



Establishment of the lime-secreting habit. The first re- 

 corded crisis, that of the assumption of the lime-secreting 

 habit perfected by animals in the Upper Cambrian, by plants, 

 marine algae, much earlier has again no accepted geologic 

 cause. The importance of this crisis is doubtless not as great 

 as that of many unrecorded ones which had gone before, its 

 significance lying mainly in the fact that the development of 

 hard parts enabled the organisms which bore them to write 

 imperishable records of their existence upon the rocks. Pre- 

 vious to this, the indications of life are indirect, such as the 

 accumulation of graphite, which is never produced in nature 

 except as the result of organic activity; or the records are in 

 the nature of the rare fossils of algae, of Protozoa (Radio- 

 laria), and of the burrows of annelid worms. It may be 

 assumed that the presence of Protozoa, on the one hand, and 

 of Annelida, on the other, in late Proterozoic sediments implies 

 the existence of all the intermediate phyla of inverte- 

 brates sponges, coelenterates, flat and round worms and the 

 probabilities are that all the other invertebrate groups except 

 the Arthropoda were also present. 



The culmination of this crisis, like several which are yet to 

 be discussed, did not come until long after its inception, for 

 while certain creatures like the Archaeocyathinae (probable 

 corals) and brachiopods show limy skeletons in the early 

 Cambrian, others were as yet chitinous, and it is not until the 

 Middle Ordovician that the process is entirely complete. 



The definite fossil record thus established shows us that 

 the evolution of great invertebrate groups occurred largely 

 before the close of the Proterozoic, hence we may not speak 

 confidently of cause and effect. Vertebrate evolution, on the 

 other hand, lies within the period of recorded evolutionary 

 history, and while there is little direct evidence regarding the 

 origin of this important phylum, the evolutionary crises 

 through which it passed are entirely within the scope of our 



