CHAPTER 1 6 



THE ANCESTRY OF THE VERTEBRATES 



The histories of the various vertebrate organs, as set forth in this 

 text, may seem to involve certain inconsistencies. Nematodes, for 

 example, appear in the account of the digestive system, but are not 

 mentioned in describing the phylogenesis of the nervous system. Simi- 

 larly, in discussing the excretory system of vertebrates, that of annelids is 

 significant, while for the phylogenesis of the nervous system that of 

 anneHds may have no meaning. The gonadic sacs of nemerteans maybe 

 genetically related to the coelomic segments of vertebrates while it is dif- 

 ficult, if not impossible, to compare their muscles with those of chordates. 



Clearly then, for the genesis of one system of vertebrate organs, the 

 condition in one group of invertebrates may be most enlightening, while 

 for another system some quite different group may reveal the ancestral 

 relations. This implies the paradoxical notion that a particular inverte- 

 brate phylum may be "ancestral" to the vertebrates for one set of organs 

 but not for another. It is as if each set of organs were an independent 

 variable in the evolution of the various phyla. For this reason, morpholo- 

 gists have ceased searching for a single common ancestor of all chordates. 



None of the several species of prehistoric men — Java, Sussex, Peking, 

 Neanderthal — appears to be directly in the line of ancestry of Homo 

 sapiens. None is "the missing link." None the less they serve to 

 bridge the gulf which separates man from his mammalian relatives. One 

 fossil type has retained certain characteristics possessed by the common 

 ancestor of both itself and Homo sapiens, which Homo sapiens has lost. 

 Another fossil type has retained a different feature. All throw some 

 Ught on the hypothetical course of evolution of modern man. All are 

 near the ancestral line. None is completely on it. Their relations must, 

 therefore, be expressed graphically by a branching diagram. 



One may, then, in considering both human origins and the larger 

 problem of vertebrate ancestry, draw quite wrong conclusions, unless 

 one keeps always in mind what is meant by "ancestor" or "ancestral 

 stage," noting always that a group may become greatly specialized along 

 one or more lines, while it retains in other organs primitive and ancestral 

 traits. When, therefore, one speaks, for example, of an annelid stage of 

 vertebrate ancestry, this should not be understood to mean that any 

 vertebrate ancestor ever was, in all respects, a typical annelid, or that 

 any annelid stands in the direct line of vertebrate descent. All that is 



628 



