HEREDITY 



179 



stages essentially similar to the embryo of the fish. But in the 

 course of development the gill slits in man and the higher ver- 

 tebrates disappear. Their position is, however, indicated by 

 the course of certain blood vessels. These follow the lines 

 blocked out in the embryo when they led to the gill slits, al- 

 though no other trace of these slits persists in the adult, and this 

 direction is not one which we could conceive as likely to have 

 arisen except for the results of inheritance from the lower ver- 

 tebrates. 



In the veins of the higher animals valves are present, so 

 arranged as to prevent the flow of blood backward and espe- 

 cially downward from the heart. In the lower animals, these 

 valves are adjusted to the position on all fours. Their adjust- 

 ment is the same in man, notwithstanding his erect posture. 

 Apparently the adjustment of the valves was completed before 

 the position on all fours gave way to the erect posture. 



FIG. 113. At left, appendix vermiformis of kangaroo; at right appendix vermiformis 



of human embryo. (After Wiedersheim.) 



In the embryo of man there exists a regular tail, supported 

 by eight distinct bones, like the tail of any other mammal. In 

 the process of development, these bones are reduced in number 

 and are joined, forming the coccyx or rudimentary tail. 



In various species of fishes, lizards, salamanders, crayfishes, 

 and other animals living in caves or buried in the ground, the 

 eyes are atrophied. Numerous cases (Fig. 114) of this sort have 

 been studied by Dr. Carl H. Eigenmann. He finds in general 

 that the young cave fish have normally developed eyes, but that 



