PHYLOGENETIC RELATIONS OF ANIMAL, GROUPS 869 



There is an extinct form of Amphibia, Stegocephalia, which shows 

 relations to the ganoid fishes and for this reason the ganoids are 

 usually named as ancestors of Amphibia. Some authors hold that 

 the lungfishes, which represent an independent branch of the Elas- 

 mobrajich group, are the ancestors of Amphibia because of their 

 ability to breathe air and live out of water. However, the former 

 view of the phjdogenetic relation between fish and amphibia is most 

 commonly held. Amphibia have well-developed bony skeleton with 

 paired appendages for locomotion on land. Lungs have appeared 

 as a means of aerial respiration, and the aortic arches have been 

 reduced to three. 



Reptiles are supposed to have descended from Stegocephalia also, 

 with most modern reptiles coming by way of Bhynchocephalia which 

 is represented by one living species, Splienodon punctatum. The 

 snakes, lizards, crocodiles, and the extinct dinosaurs have probably 

 branched from this group, while the turtles are thought to have de- 

 scended through Theromorpha, another extinct branch of Stego- 

 cephalia. The dinosaurs are credited with the ancestry of birds by 

 way of a toothed, feathered, extinct form known as Archaeopteryx. 

 It was essentially a flying reptile. The mammals probably descended 

 from the reptilian group Theromorpha by way of our modern mono- 

 tremes which lay eggs, hatch them out, and then suckle the young 

 with milk from mammary glands. The marsupials, such as kan- 

 garoos and opossums, are next in order, and from these it is thought 

 the Placentalia have arisen. "Within this group some authorities 

 hold the view that the Primates, the order including man, have 

 arisen from Insectivora. The apes and monkeys belong to the Pri- 

 mate group, and there has been some misunderstanding among lay- 

 men generally in regard to the possible relationship of man and the 

 apes. Most people have the misconception that this is a linear 

 descent in which the most advanced member of the lower group 

 represents the immediate ancestor of the next higher group. As 

 a matter of fact, the theory is not that the higher monke3''s are in 

 the process of becoming apes and the higher apes becoming men, 

 but that all three of these groups have had origin as different lines 

 from a common primitive form. 



Recapitulation Theory. — In the early part of the nineteenth cen- 

 tury von Baer observed that the early stages of vertebrate embryos 

 of different classes had a very close resemblance to each other. He 



