158 W. K. BROOKS. 



Walcott's collection of 141 American species from the lower Cam- 

 brian is distributed over most of the marine groups of the animal 

 kingdom, and, except for the absence of the remains of vertebrates, 

 the wliole province of animal life is almost as completely covered 

 by these 141 species as it could be by a collection from the bottom 

 of the modern ocean. 



Four of the American species are sponges, two are hydrozoa, 

 nine are actinozoa, one an echinoderm, twenty-nine are brachiopods, 

 three are lamellibranchs, thirteen are gasteropods, fifteen are ptero- 

 pods, eight are Crustacea, fifty-one are trilobites, and the trails and 

 burrows show the existence of at least six species of bottom forms, 

 probably worms or Crustacea. 



The most noteworthy characteristic is the completeness with which 

 these new species outline the whole fauna of the modern sea-floor. 



Nothing brings home more vividly to the zoologist a picture of 

 the diversity of the lower cambrian fauna and of its intimate re- 

 lation to the bottom fauna of to-day than the thought that he would 

 have found, on the old cambrian shore, about the same opportunity 

 to study the embryology and anatomy of pteropods, gasteropods 

 and lamellibranchs and crusfacea and medusae that he now has at 

 a marine laboratory, and that his studies in phylogeny would have 

 had about the same form then that they have now. 



Biological evidence based on embryology and anatomy and on 

 the habits and affinities of animals is justly regarded, by zoologists 

 at least, as a more perfect record of the early history of life than 

 palaeontology, and we accept, without question, proofs of j)hylogeny 

 which refer to a time very much more remote than the age of the 

 oldest fossils. 



We must not forget, however, that our generalizations in primi- 

 tive phylogeny rest for the most part on the study of swimming 

 or floating larvae of minute size and simple structure, which we 

 can have little hope of finding as fossils. 



In the formations which follow the lower cambrian, species 

 gradually become more numerous, but this is due to divergent 

 specialization, and Walcott says that if a comparison be made 

 between the olenellus zone (lower cambrian) and the silurian 

 fauna, the superiority of the latter in number of species, genera 

 and families is at once apparent. 



