744 General and Applied Biology 



tures, and such differences or variations as exist are principally the result 

 of the absence of some minor part or the transformation of a certain part, 

 depending on disuse or specific use to which that part has been put. 

 Nearly all the bones, muscles, nerves, and blood vessels are constructed 

 and arranged in a homologous manner in the forelimbs of the entire 

 group from the lower types or frogs to the higher types or man. The 

 same thing is true for the hindlimbs, digestive systems, reproductive sys- 

 tems, and circulatory systems of this series. 



b. From Vestigial Structures: Man has approximately one hundred 

 useless or harmful structures which are also represented in lower types, 

 in which case the same kind of structure may be very useful. What is 

 the explanation of the presence of such useless or harmful structures in 

 certain animals if they have not had their origin in some common an- 

 cestor? Why should they have been specifically placed in animals in 

 which they are useless? Is it not logical to believe that they have evolved 

 differently in different animals? 



Illustrations of vestigial structures in man are quite numerous, but the 

 following are most commonly cited as being representative: (1) The 

 vermiform appendix is a remnant of an organ which is useful in certain 

 herbivorous (plant-eating) animals. The appendix may have had a 

 specific function in man many generations ago, although no specific 

 function can be stated for it at present. (2) The third eyelid in the 

 inner angle of the human eye corresponds to the nictitating membrane 

 or lid which moves laterally across the eye in such lower animals as the 

 frog, bird, and dog. (3) Muscles of the external ear are useless for man 

 but are used by lower animals to turn the ear in the proper direction to 

 acquire the sound waves more accurately. (4) The terminal vertebrae 

 (coccyx) are of no value to man, but they are the foundation for the 

 external tail in lower animals. It is interesting to note that the early em- 

 bryos of man (Fig. 363) possess an external tail which is useless and 

 which is discarded before the adult stages are reached. Only occasionally 

 does the external tail persist in the adult man. (5) The lobe of the ear 

 is of no practical benefit to man, although it may have had some func- 

 tion in the past. (6) The point of the ear, known as Darwin's point, 

 on the edge of the upper roll or margin of the human ear, corresponds 

 to the tip of the ear of animals who hold their ears upright. What is 

 the value of this vestigial structure? Why should it have been placed 

 there specifically if there is not some type of relationship? 



