192 THE DARWINIAN THEORY 



made towards the determination of the ancestry of 

 Vertebrates. This suggests that the Vertebrate 

 stem arose very early, and must not be derived from 

 groups such as annelids, lobsters, spiders, and sea- 

 urchins, each of which has been claimed as the 

 (ancestor of , Vertebrates in recent years. Any 

 \attempt to get further back than this is mere specu- 

 lation. The only available evidence is that of 

 embryology, and that this is trustworthy we have 

 abundant proof as regards the later stages of 

 development. The Ascidian tadpole is shown by 

 Appendicularia to be a possible adult, and indeed to 

 correspond very closely to an existing adult animal. 

 With regard to the earlier stages of development we 

 have seen in a former lecture* that this will also 

 apply, and that adult organisms are known corre- 

 sponding to the several stages in the early develop- 

 ment of Amphioxus which may possibly represent 

 ancestral forms. 



Thus, by the aid of anatomy, palaeontology, and 

 embryology we are able to define fairly clearly 

 the broad lines of Vertebrate ancestry ; and the 

 conclusion we arrive at is the usual one, viz., to 

 emphasize the uselessness of search for directly 

 intermediate links between existing forms, and 

 to drive back the origin of this, the highest 

 group of animals, to times earlier than those of the 

 oldest fossiliferous rocks ; to show that our only 

 hope of obtaining information concerning these first 

 progenitors depends on the extent to which their 

 existing descendants have preserved the record in 



* See page 114. 



