520 



GENERAL CONSIDERATIONS 



arches (Fig. 245), the gradual development of the vertebral column to 

 replace the primitive notochord, and the development of the different 

 types of excretory system (Fig. 216), all of which have been previously 

 noted. 



Paleozoology offers very many evidences that anim.al life has gradually 

 changed. Apparently no rock strata exist which show traces of the 



A B C D E F G 



Fig. 318. — Parallel stages in the development of several vertebrates. A, fish; B, 

 salamander; C, turtle; D, chick; E, pig; F, calf; G, rabbit; H, man. In each series, a is an 

 early stage, showing the pharyngeal slits; b, a later stage, in which the first two have 

 developed gills and the last six show the pharyngeal slits disappearing and limits and tails 

 developing; c, a still later stage, in which the differences between the reptile and bird on the 

 one hand and the mammals on the other have become pronounced, marked resemblances 

 between those of each group persisting. {From Guycr, ''Animal Biology," after Haeckel, 

 through Romunes.) 



earliest life. This is due to metamorphism (Sec. 583). The older rocks 

 give only indirect evidence of the existence of life on this earth. The 

 earhest of the sedimentary rocks to contain an abundance of fossil types 

 belong to the Cambrian period. Since that time strata deposited, one 

 after another in order of time, show the appearance of many types which 

 have gradually increased in numbers and variety, have reached a climax, 

 then have declined, and finally have become extinct. Others have been 

 able to maintain themselves to the present. But there has been a steady 



