212 GEOLOGICAL BIOLOGY. 



brian era. It is difficult to appreciate how far back in the 

 -world's history this shifis the great events of evolutional 

 •elaboration, and how little it leaves to be accomplished with- 

 in even the immense periods of geological time of which we 

 liave the least trace of the history of organisms. 



Modern Zoology applicable to the Fauna of the Cambrian Era. 

 — During the preparation of these pages the writer took occa- 

 sion to examine the details of form and structure discussed in 

 the lectures of a well-known professor of Invertebrate Zoology. 

 It was found that, so far as the evidence is preserved, the 

 great majority of the differentiations which are considered in 

 such a course of lectures were actually present in the Cam- 

 brian era. What has taken place since is differentiation in 

 respect of less important characters. In other words, a pre- 

 liminary course of lectures on Invertebrate Zoology (eliminat- 

 ing the animals adapted to aerial and pure fresh-water envi- 

 ronment) would be adapted to the fauna of the Cambrian 

 era. This statement will probably surprise the reader to 

 Avhom it comes now for the first time. It is certainly a most 

 remarkable fact that the great plan-work of structure of all 

 the invertebrates was so fully elaborated at such an extremely 

 early period, and that since that time, for the millions of 

 years that have followed, the modification in organic forms 

 has been so slight. It is more impressive than the fact that 

 several genera of Brachiopods (Lingula, Discina, etc.) living 

 to-day w^ere represented in the Cambrian by forms separable 

 from them only by the closest scrutiny. 



Characters whose Origin is Traced Back to Cambrian Time. — 

 Assuming the correctness of the above statements, the in- 

 quiry may be made, What are the characters, expressed con- 

 tinuously up to the present, which made their first appearance 

 in Cambrian time ? 



First, there is a branch, called Protozoa, all the animals of 

 which are relatively minute, some of them truly microscopic; 

 their bodies are composed of a jelly-like substance, called 

 protoplasm, without cellular differentiation, and void of per- 

 manent specialization of function. They show great bodily 

 activity, but in no permanent direction. The whole sub- 

 stance of the body seems transiently to be experimenting in 



