340 STRUCTURE OF THE VERTEBRATES 



cestral to another, except in the inexact sense that one may be 

 much nearer the common stem. A distant cousin could not be an 

 ancestor, and it is well to keep in mind that in actual number 

 of years each evolutionary line is as old as any other. Some of 

 the lines have remained much more like the early stock than 

 others, and may be used as representatives of the common stem. 

 Amphioxus is an animal of this type. The present members of 

 the cephalochordate group have many specializations which 

 have been added as adaptations to changing conditions; but in 

 fundamental anatomy and embryological development they are 

 remarkably generalized in nature, and form a perfect beginning 

 for the family tree of the vertebrates. 



Only fairly generalized animals will exist through violent 

 environmental changes. Many specializations fit a group for 

 only one type of life, and here lies one of the reasons for the 

 great number of races which have evolved and then passed 

 completely from the earth. But when the dominant animals of 

 the time become extinct, or of no great importance, some ap- 

 parently insignificant race survives and gives rise to some new 

 dominant type. To this class belong the mammal-like reptiles. 

 Small, rapidly moving, seemingly unfit for existence when in 

 competition with the dinosaurs, they were the parent stem of 

 the mammals which survived a revolution of the earth's surface 

 and became the dominant life of the following era. 



The diagrams (pp. 337-8) show the major groups of verte- 

 brates, and the lines from which they came. Some of the lines 

 are left unconnected, to indicate that the earliest ancestors are 

 not known, and the period in which they arose is undetermined. 

 This is more true of the earlier lines, both for the reason that 

 they were less likely to be preserved, and the older the stratum 

 the more alteration there would be in the fossil bearing rocks. 



A. Origin of the Vertebrates 



In the first two chapters is an outline of the Chordate theory 

 of vertebrate evolution. This was included there in view of the 

 fact that the pre-vertebrates are of undoubted chordate rela- 

 tionships, and because the author feels that this is by far the 

 best established hypothesis of vertebrate evolution. 



