76 VERTEBRATE ZOOLOGY 



the known lower groups. It is generally believed, for example, that 

 the Arthropoda are descended from the Annelida, the Annelida from 

 the Platyhelminthes, the Platyhelminthes from the Trochelminthes, 

 etc. It is likely to be forgotten that these groups as we know them, 

 both recent and fossil, are highly specialized, and that a highly special- 

 ized group is not likely to retain sufficient plasticity to give origin to 

 any radically new departures that might lead to new phyla. The mod- 

 ern phylogenist has come to believe that the early ancestors of such 

 large fundamentally isolated groups as the phyla all originated far 

 back in pre-Cambrian tunes and are therefore forever lost to us as 

 actual relics. Nevertheless there are still extant some theories that 

 derive the vertebrates from various contemporaneous invertebrate 

 phyla. If the vertebrates did come off from any known invertebrate 

 types, what more natural than to look to the other great metameric 

 phyla for the ancestral conditions? The only truly metameric phyla 

 among the invertebrates are the Annelida and the Arthropoda and 

 there are two rival theories, one claiming that the annelids and the 

 other that the arthropods are the ancestors of the vertebrates. 



Both theories base their argument on fundamental morphological 

 resemblances between the vertebrate and the annelid, or the arthro- 

 pod, as the case may be. The annelid is unquestionably more general- 

 ized in its organization than the vertebrate, and it is true that the 

 vertebrate embryo much more closely resembles the annelid than does 

 the vertebrate adult. There are many striking homologies between 

 all three groups (annelids, arthropods, and vertebrates) and nothing 

 could be further from the thought of the writer than to deny that any 

 phylogenetic relationship exists between them. Such a denial would 

 be equivalent to a denial of the validity of the whole principle of ho- 

 mologies, upon which the science of morphology rests. An admission 

 of relationship between annelid and vertebrate or between arthropod 

 and vertebrate is, however, quite different from an admission that the 

 vertebrate is descended from either annelid or arthropod. Is it not 

 much more reasonable to suppose that all three of these highly special- 

 ized groups, that have so much in common, have been derived from a 

 primitive ancestor characterized by the features that all three groups 

 have in common? Such an ancestor would be metameric, ccelomate, 

 with antero-posterior, dorso-ventral, and bilateral axes, with prob- 

 ably ciliary bands as a mode of locomotion, with tubular nephridia, 

 with well-defined double nerve-cord and paired ganglia, and possi- 



