2l8 GEOLOGICAL BIOLOGY. 



nerves and organs of sense ; Siistcntation, as exhibited in 

 organs of alimentation, digestion, nutrition, circulation, and 

 purification ; Reproduction, with special organs and separation 

 of sex. 



Insignificance of Characters of Marine Invertebrates Evolved 

 since Cambrian Time. — ^When we would speak of evolution of 

 different kinds of organisms, it is not regarding the evolution 

 of the difTerences above described that the geologist has any 

 evidence ; they were present at the beginning of the records. 

 All this had been accomplished when we get the first glimpse 

 of the earliest known relic of an organism. The simplest 

 types of organisms are living to-day, as are the most elabo- 

 rated types ; but when we go back to this earliest page of 

 geological history we find (with the exception of vertebrates) 

 all the grand types of animals already living together. So far 

 as these grander differences of organization are concerned, the 

 millions of years of geological time throw no light upon the 

 way by which they came about. 



When we consider that our knowledge is only of marine 

 organisms, and how extremely meagre is the evidence we have 

 of them, it becomes highly probable that for animals adapted 

 to this environment nothing of branch, class, or possibly of 

 ordinal rank has been evolved since Cambrian time. 



