THE ANCESTRY OF THE VERTEBRATES 



665 



dates, and not inherited from metameric invertebrates. Hesitation to 

 accept this conclusion seems to be due chiefly to the fact that no pre- 



HOMO 



SINANTHROPUS 



AUSTRALOPITHECUS- 



PROPLIOPITHECUS« 



INSECT IVOR 



MONOTREMES- 

 STEGOCEPHALA 

 DIPNOI 



MOLLUSCS 

 ARTHROPODS 



ANNELIDS 



MOLLUSCOIDS 



ROTIFERS 



THREADWORMS 



CYCLOSTOMES 

 .UROCHORDS 



FLATWORMS 



DRYOPITHECUS 

 LEMUROIDS 



'MARSUPIALS 



•THEROMORPHS 



CROSSOPTERYGIANS 



ELASMOBRANCHS 



CEPHALOCHORDS 

 HEMICHORDS 



ECHINODERMS 



OELENTERATES 



SPONGESv 



PROTOZOA 



Fig. 540. — A diagram of the phylogenesis of man, based on the assumption that the 

 Protochordates resemble vertebrate ancestors more closely than do the annelids. 

 Annelids and other Proterostomians have, it is assumed, diverged from the main line 

 of vertebrate ancestry very early in animal phylogenesis. Before man emerged in the 

 late Tertiary transitional forms between man and the lower Primates made their 

 appearance. Among these the best known are Propliopithecus, Dryopithecus and 

 Australopithecus, which are true "connecting links." 



chordates can be found which resemble larval chordates. Thus the 

 chordate clue seems to lead us into a blind alley out of which the most 

 promising exit is the way back. 



