SOME PROBLEMS OF ANNELID MORPHOLOGY. 55 



Now, under the evolutionary interpretation of nature, 

 every higher and more complex form has arisen from a 

 lower and simpler one, presumably now extinct, but 

 possibly more or less similar to forms still existing. In 

 any case the key to the genealogy of higher forms must 

 be sought in the organization of lower but related forms. 

 In morphology, as in every field of research, the inter- 

 pretation of complex phenomena must be sought through 

 the study of simpler phenomena. And so it comes about 

 that the indispensable basis for every inquiry respecting 

 the derivation of vertebrates is an accurate knowledge of 

 those invertebrate forms, if any there be, that possess any 

 features in common with the vertebrate type. I scarcely 

 need to add that no zooloirist would look for the actual 



o 



progenitor of vertebrates among existing invertebrates. 

 We seek only for forms more or less nearly similar to 

 the ancestral proto-vertebrate. The character of the 

 ancestral type must be largely a matter of inference, not 

 of direct observation. 



Now, what are the most fundamental and interesting 

 features of the vertebrate body.^ Let us leave aside 

 such characters as the presence of a corda dorsalis, the 

 relation of the central nervous system to the alimentary 

 canal, and the double-tubular composition of the body — 

 all of which are peculiar to the vertebrates or their im- 

 mediate allies. Let us consider only those broader 

 characteristics on which the distinctive vertebrate feat- 

 ures are, as it were, moulded. 



I think most morphologists will agree that the most 

 striking feature of the vertebrate body as regards struc- 

 ture is its metamerism (or segmented structure) ; and 

 its most remarkable feature as regards development is 



