SALPA IN RELATION TO EVOLUTION OF LIFE. 155 



or lamellibranchs, as they are from those of the present day. The 

 eiForts of anatomists and embryologists to reconstruct the primary 

 phylogeny of the raetazoa have so far yielded few trustworthy 

 results, and the results which are most trustworthy are usually 

 those which are the most indefinite. 



We are therefore forced to believe that the early steps in the 

 establishment of the various types of metazoa were taken under 

 conditions which had some essential difference from those which 

 have prevailed, without any fundamental changes, from the time 

 of the oldest fossil to the present day ; and we are also forced to 

 believe that most of the great lines of descent were represented at 

 some time in the remote past by ancestors which, living a different 

 sort of life, differed essentially in structure as well as habits, from 

 the representatives of the same types which are known to us. Fur- 

 thermore, embryology teaches that each great group still bears in- 

 ternal evidence of descent from pelagic ancestors, and while the 

 characteristics of these ancestors are in most cases unknown, a few, 

 like appendicularia, are still found alive. 



Our knowledge of the evolution of the metazoic types has certain 

 general features which are essentially the same for all, but each 

 group has also in its history much that is individual, and any 

 general statement requires so much qualification that the history 

 of an illustrative group is more instructive than a general summary. 



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

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

 modifications, 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 the plan of 

 structure is always the same. Palaeontology gives us most valua- 

 ble evidence regarding the course of evolution within the limits of 

 a class as in the crinoids and in the echinoids ; but we appeal to it 

 in vain for light upon the organization 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 ani- 

 mals, palaeontology is silent, and throws them back upon us as 

 unsolved riddles. 



