214 PROCEEDINGS OF THE ACADEMY OF [April, 



(lure of the iihylofrcnist. A character which leads far back into the 

 ancestry is the guiding line in interpretation of descent. If, 

 through the continuous change which a race of organisms undergoes 

 during the process of evolution, a particular part becomes much less 

 rapidly changed than the others, that part certainly represents the 

 ancestral characters most fully, and for the phylogenist its persis- 

 tence or conservatism is the criterion of its morphological value. A 

 character which persists through a very long racial period must do 

 so by virtue of being of particular value for the economy of the 

 organization, or for the perpetuation of the race. Structures of 

 less value are more readily modified, or substituted, or even lost. 



It is then clear that the first step to take in determining the char- 

 acters on Avhich classification should be based, is to find out the rela- 

 tive stability or conservatism of these characters, at the same time 

 recalling that no characters should be neglected in the examination, 

 but that they be ascribed value in the order of their conservatism. 



The ever-recurring problem to the classifier is that of determin- 

 ing the interrelationships of, e. g., these organisms. A, 5 and G, 

 when A agrees with B in character d, but differs from B in charac- 

 ter e, while agreeing with C m character e but differing from C in 

 character d: is the value of character d of greater or less value 

 here than that of character e f The answer may be a purely arbi- 

 trary one, such as Avould best suit the classifier's preconceived views; 

 or it may be made less arbitrary and more logically precise by the 

 application of the principle of conservatism as denoting value. 



The application of this principle would seem at first sight very 

 simple, but in practice it has been found very difl^cult. For in a 

 racial progress, as Kleineuberg has shown so Avell for the individual 

 development, one organ may become gradually substituted for 

 another, so that it is diflicult to decide at what point the first 

 disappears and the second takes its place. A case in point is the 

 substitution of one kidney system for an earlier one, and of a third 

 for the second, in the evolution of the Vertebrates. In this case 

 a persistence of the first kidney (pronephros) is found even when 

 the second kidney (mesonephros) has been substituted in its place, 

 and even in the organization showing the substituting third kidney 

 (metanephros) a small portion of the first is still retained. The 

 case becomes more difficult to interpret when not separately arising 

 parts succeed each other, but Avhen in the progress of the race 



