614 



COLLEGE ZOOLOGY 



v^^ Nictitating 



membrane 



Man 



Bird 



Coccyx 



Child with 

 a tail 



Figure 434. Vestigial organs are apparently useless remains of organs found in some animals, 

 which are represented by useful structures in others. Eyes of man and bird, showing functional 

 nictitating membrane (third eyelid) in the bird, and nonfunctional vestigial nictitating mem- 

 brane in man. Vermiform appendix of man, a useless structure with a bad reputation, is not 

 now regarded as a vestigial organ by some; the corresponding organ in herbivorous mammals 

 apparently functions in digestion. Coccyx of man represents the bony remnants of the tail of 

 lower animals, but an occasional child is born with a fleshy tail several inches long. 



Yolk sac 



Shark 



Mar> 



Figure 435. Gill slits in embryos of animals belonging to three different classes of vertebrates. 

 We find gill slits in adult fish, but they are present only in embryonic birds and mammals. 



favor of the principle of organic evolution. 

 For example, studies have been made of the 

 crystals formed by the hemoglobin of the 

 blood, and comparisons show that the crys- 

 tals of closely related species are more 

 nearly alike in form than those of distantly 



related species. Thus the crystals in the 

 blood of species belonging to one genus re- 

 semble each other more closely that they do 

 those of species belonging to other genera. 

 The relations of families, orders, etc., can 

 also be determined in this way; for example, 



