458 THE JOURNAL OF GEOLOGY. 



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 embryology 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 backwards as we can through the study of fossils, we still find 

 these lines distinctlv laid down. The lower Cambrian Crustacea, 

 for example, are as distinct from the lower Cambrian echino- 

 derms 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 ancestors which, living a different sort of life, differed 

 essentially, in structure as well as in habits, from the representa- 

 tives of the same types 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 structure in any other group of animals. All through the 

 series of fossiliferous rocks echinoderms are found, and their 

 plan of structure is always the same. Palaeontology 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 organization of the primi- 

 tive echinoderm, or for connecting links between the classes. 

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



