786 



EVOLUTION AND CONSERVATION 



Part VI 



tebrae, gill slits, aortic arches, and two-chambered heart that exist for different 

 lengths of time. 



The top Hne of drawings in figure 38.7 shows young embryos with funda- 

 mental structures that are common to all vertebrates — a striking presentation 

 of similarities. Figures in the next line show that the body form of the fish is 

 not shared by the cow. Finally, in each of the oldest embryos there are one or 

 more structures that are unique, the shell of the turtle, wing of the bird, snout 

 of the pig, and the domed head of man. Unity is apparent in the younger 

 embryos. Diversity is striking in the older ones. 



Vestigial Organs. These are small, useless vestiges of structures that may be 

 well-developed and functional in near kin and ancestors of their owners. They 

 are maintained by heredity and the conservatism of living matter — treasured 

 clocks that have stopped ticking. The human body has a collection of several 

 dozen such structures. Among them are the ear muscles so feeble in man, so 

 active in horses; the vermiform appendix, a nuisance in man, a digestive cae- 

 cum in rabbits; and the nictitating membrane, a little fold of flesh in the inner 

 angle of the human eye, a protective membrane that may instantly slip over 

 the eye of a bird. 



Fish 



SaioAtuindcr Tortoise 



Fig. 38.7. Vertebrate embryos showing the inheritance of a unified basic plan 

 followed by diverse structure which easily identify the animals. Embryos in three 

 successive and comparable stages of development. Upper row, all are in general 

 much alike. Middle, lower vertebrates, fish and salamander show difi'erences 

 sooner than mammals, pig to man. Bottom, all types are recognizable. These figures 

 originally after Haeckel lack detail and certain points of accuracy but they excel 

 in emphasizing essential agreements and ultimate differences. (Courtesy, Pauli: 

 The World of Life. Boston, Houghton Mifflin Co., 1949.) 



