Chapter 6 

 ELEMENTS OF VERTEBRATE PHYLOGENY 



If one knows something of the history of a group of animals and 

 its position in the animal kingdom, one may more easily draw correct 

 conclusions as to how it acquired its characteristic morphology. We 

 may learn of some structure in the eye of one of the lower animals which 

 looks intriguing as a possible forerunner of some detail of the human 

 eye; but we need to know whether the group that exhibits the structure 

 in question is anywhere near the main line of evolution, or represents 

 a blind alley from which nothing higher than itself has ever emerged. 



A little about vertebrate group inter-relationships is therefore included 

 here, that the reader may better understand why one animal has solved 

 a given visual problem in one way while another, given other raw 

 materials, has had to find a different — perhaps better, perhaps poorer — 

 solution to the very same problem. In devising adaptive structures, each 

 animal group has had to do what it could with the materials at hand — 

 the assemblage of characteristics and structures with which the group 

 happened to be endowed when it crystallized out of the stream of life. 

 May we reasonably look to the teleost fish for the prototype of some 

 amphibian ocular structure? Can we expect to see in the snakes a feature 

 which the lizards discarded? Can we fairly compare the human eye more 

 closely with the eye of a salamander, or with that of a bird? A brief 

 review of the vertebrate pageant will help the reader to answer such 

 questions as they arise during his study of subsequent chapters. 



At the bottom of the vertebrate scale stand the cyclostomes; and just 

 above them, the many types of true fishes. From one of these types the 

 first land animals, the ancient amphibians, were derived. They in turn 

 gave rise on the one hand to the modern amphibians and on the other 

 to the reptiles. The reptiles differentiated into a large number of orders, 

 only four of which have persisted to the present day. From one group 

 of extinct reptiles came the birds; and from another (much older) group, 

 the mammals — warm-bloodedness and heat-retaining coverings (feathers, 

 fur) thus having originated independently in the two highest classes of 

 vertebrates. 



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