THE ORICxIN OF THE OLDEST FOSSILS. 3G1 



sncli a character as to indicate that it is a decided and unmistakabh' 

 approximation to the primitive fauna of the bottom, bej^ond whicli life 

 was represented only by minute and simple surface animals not likely 

 to be preserved as fossils. 



liJ'othing brings home more vividly to the zoologist a picture of the 

 diversity of the Lower Cambrian fauna and of its intimate relation to 

 the fauna on the bottom of the modern ocean than the thought that 

 he would have found on the old Cambrian shore the same opportunity 

 to study the embryology and anatomy of pteropods and gasteropods 

 and lamellibranchs, of Crustacea and medusre, echinoderms aud bra- 

 chiopods, that he now has at a marine laboratory; that his studies 

 would have followed the same lines then that they do now, and that 

 most of the record of the past which they make known to him would 

 have been ancient history then. Most of the great types of ancient 

 life show by their end:>ryology that they run back to simple and minute 

 ancestors which lived at the surface of the ocean, and that the common 

 meeting point must be projected back to a still more remote time, before 

 these ancestors had become differential from each other. 



After we have traced each great line of modern animals as far back- 

 ward as Ave can through the study of fossils, we still find these lines 

 distinctly laid down. The Lower Cambrian Crustacea, for example, 

 are as distinct from the Lower Cambrian echinoderms or pteropods or 

 lamellibranchs or brachiopods as they are from these of the present day, 

 but zoology gives us evidence that the early steps in the establishment 

 of these great lines were taken under conditions which were essentially 

 different from those which have prevailed, without any essential change 

 from the time of the oldest fossils to the present day, and that most of 

 the great lines of descent were represented in the remote past by ances- 

 tors, which, living a different sort of life, differed essentially, in struc- 

 ture as well as in habits, from the representatives of the same ty])es 

 which are known to us as fossils. 



In the echinoderms we have a well-defined type represented by 

 abundant fossils, very rich in living forms, very diversified in its 

 modification, and therefore well fitted for use as an illustration. This 

 great stem contains many classes and orders, all constructed on the 

 same plan, which is sharply isolated and quite unlike the plan of struc- 

 ture in any other group of animals. All through the series of fossilifer- 

 ous rocks echinoderms are found, and their plan of structure is always 

 the* same. Paleontology gives us most valuable evidence regarding 

 the course of evolution within the limits of a class, as in the crinoids 

 or the echinoids; but we appeal to it in vain for light upon the organiza- 

 tion of the primitive echinoderm or for connecting links between the 

 classes. To our questions on these subjects, and on the relation of 

 the echinoderms to other animals, paleontology is silent, and throws 

 them back upon us as unsolved riddles. 



