THE TAXONOMIC SERIES 389 



intermediate grade. This series is fairly well represented by 

 almost any scheme of classification adopted by systematists 

 to-day. Great similarity in form and structure naturally 

 suggests a blood relationship and a common origin. But the 

 taxonomic series represents a continuous chain of such relations, 

 and hence leads to the conclusion that the entire animal king- 

 dom had a common origin, and that by a process of evolution 

 the higher forms have developed from lower forms as the com- 

 plex adult develops from the simple egg. 



791. The taxonomic series is, however, not adequately 

 represented by a line and it is now more frequently compared 

 to a tree. At its base this genealogical tree divides into two 

 trunks, one representing the vegetable kingdom, the other the 

 animal. The base from which they both spring represents the 

 many unicellular forms which have both vegetable and animal 

 characteristics. Then up along the animal trunk come the less 

 differentiated forms, like some Ccelenterates, Annelids, Peri- 

 patus, Branchiostoma, etc. The branches represent the highly 

 differentiated forms and spring from the main stem at various 

 points; the Sponges below, higher, the Echinoderms, Arthro- 

 pods, Molluscs, etc., while several large branches at the top 

 represent bony Fishes, Reptiles, Birds and Mammals. 



792. With the advancement of the study of anatomy much 

 information has been obtained which throws light on this ques- 

 tion. In the vertebral column of Vertebrates, for example, 

 we have a structure which indicates clearly a relationship 

 between the five classes of Vertebrates, but when we examine 

 the skull and the appendages we seem at first to' have only 

 hopeless diversity. But here also a wonderful uniformity 

 appears on more careful investigation. With regard to the 

 skeletal portions, the wing of a bird, the fore limb of a bat, the 

 flipper of a whale, the fore legs of the horse and dog and the arm 

 of man, are all constructed of the same elements and each one 

 of these appendages may be homologized bone for bone with 



