THE MEANING OF EVOLUTION 



253 



places where lines of descent ought to converge. They 

 show us, for instance, that the oldest Birds known 

 were decidedly reptilian in their morphology, but they 

 do not show us an animal which was neither Bird nor 

 Reptile, but from which both groups of Vertebrata 

 have descended ; and this is almost always the case in 

 our hypothetical schemes of phylogeny. Morphology 

 has continually to postulate the existence of " an- 

 nectant " forms, " Archi- Mollusc," " Protosaurian," 

 " Protochordate," etc. : hypothetical animals which 

 combine the characters of 

 those which lie near the 

 bases of diverging lines of 

 descent. There is nothing 

 to guide us in the con- 

 struction of these annectant 

 forms except the progres- 

 sive simplicity of structure 

 indicated in the morpho- 

 logical and palaeontological 

 series. The earlier Birds 

 had teeth, for instance, and 

 so have the Reptiles, there- 

 fore the annectant form had teeth, and it was an 

 animal combining the schematic morphology of both 

 Birds and Reptiles. But just according to the value 

 which we attach to one morphological character 

 rather than another, so will the structure of the 

 annectant form differ. Is, for instance, the alimentary 

 canal of the Vertebrate the most fundamental and 

 conservative part of its morphology : that is, is it the 

 structure which has been most resistant to change in 

 the course of the evolutionary process ? Then we may 

 regard the Vertebrates as having descended from some 

 animal which was closely related to the Annelid worms. 



Archi- Form 



Fig. 25. 



