FOSSIL PEDIGREES 121 



discoveries will unquestionably force us to admit that many 

 of the -species now classed as ''extinct" are in reality living 

 forms, which must be added to our list of "persistent types." 

 "It is by no means improbable," says Heilprin, "that many 

 of the older genera, now recognized as distinct by reason of 

 our imperfect knowledge concerning their true relationships, 

 have in reality representatives in the modern sea." {Op. cit. 

 pp. 203, 204.) 



Finally, the whole of our present taxonomy of plants and 

 animals, both living and fossil, stands badly in need of revision. 

 Systematists, as we have seen in the second chapter, base 

 their classifications mainly on what they regard as basic or 

 homologous structures, in contradistinction to superficial or 

 adaptive characters. Both kinds of structure, however, are 

 purely somatic, and somatic characters, as previously ob- 

 served, are not, by themselves, a safe criterion for discrim- 

 inating between varieties and species. In the light of recent 

 genetical research, we cannot avoid recognizing that there has 

 been far too much "splitting" of organic groups on the basis 

 of differences that are purely fiuctuational, or, at most, muta- 

 tional. Moreover, the distinction between homologous and 

 adaptive structures is often arbitrary and largely a matter 

 of personal opinion, especially when numerous specimens are 

 not available. What the "Cambridge Natural History" says 

 in allusion to the Asteroidea is of general application. "While 

 there is considerable agreement," we read, "amongst author- 

 ities as to the number of families, or minor divisions of un- 

 equivocal relationship, to be found in the class Asteroidea, 

 there has been great uncertainty- both as to the number and 

 limits of the orders into which the class should be divided, 

 and also as to the limits of the various species. The 

 difficulty about the species is by no means confined to 

 the group Echinodermata ; in all cases where the attempt is 

 made to determine species by an examination of a few speci- 

 mens of unknown age there is bound to be uncertainty; the 

 more so, as it becomes increasingly evident that there is no 



