THE EVIDENCE FOR EVOLUTION 



727 



Such a series o£ geographically distributed subspecies is called a Rassen- 

 kreis (German, race-circle). 



The classification of living organisms into well defined groups is 

 possible because most of the intermediate forms have become extinct. 

 If representatives of every type of animal and plant that have ever lived 

 were still living today, there would be many series of intergrading forms 

 and the division of these into neat taxonomic categories would be diffi- 

 cult indeed. The present-day species have been compared to the terminal 

 twigs of a tree whose main branches and trunk have disappeared. The 

 fascinating puzzle for the taxonomist is to reconstruct the missing 

 branches and put each twig on the proper branch. 



311. The Evidence from Anatomy 



Comparisons of the anatomy of different animals have been made 

 throughout this text. In each instance it was found that if we study the 

 details of the structure of any particular organ system in the diverse 

 members of a given phylum, it is clear that there is a basic similarity 

 of form which is varied to some extent from one class to another. The 

 skeletal, muscular, circulatory and excretory systems of the vertebrates 

 provide especially clear illustrations of this principle, but this is gen- 

 erally true of all systems in all phyla. You w'lW recall that not all simi- 

 larities can be used in classification, but only those based on homologous 

 organs (p. 424), ones which are basically similar in their structure, in 

 their relationship to adjacent structures, in their embryonic development, 

 and in their ner\'e and blood supply. A seal's front flipper, a bat's wing, 

 a cat's paw, a horse's front leg and a human hand, though superficially 

 dissimilar and adapted for quite dilierent functions, nevertheless are 

 homologous organs. Each consists of almost the same number of bones, 

 muscles, nerves and blood vessels arranged in the same pattern, and their 

 mode of development is very similar. The existence of such homologous 

 organs implies a common evolutionary origin. 



Wisdom tooth 



•Plica 

 semilunaris 



NippUe in 

 maJe,l 



Pyratnidalis nasi muscles 



Ji ■^•|Lnf':^;^ndinous J 



1' iwl p .-.W inscraptionsU ^l 

 l|it!')''"i\ p'*rof Rectus (/ 

 V \i'< M abdominis! 

 \\M ' ij muscle \ 



mml muscle. 



Coccygeal 

 vertebrae 



Figure 35.4. Diagrams of some of the vestigial organs of the human body. 



