64 E VOL UTION AND DISEASE. 



useful purpose, are extremely variable, occur in both 

 sexes, and often are of large size (fig. 31). These 

 auricles, as will be shown in detail subsequently, are 

 enlarged opercula, yet an enormous space of time has 

 elapsed since the gill-slit they guarded were functional. 

 Nevertheless they illustrate the view I am advoca- 

 ting, for gill-slits and opercula were of high functional 

 importance in the ancestors of mammalia, and are still 

 conspicuous in the early embryo. As cervical ears or 

 auricles will occupy our attention at some length 



presently, a few examples of 

 vestigial structures, and the 

 mischief which they now and 

 then occasion, will be con- 

 sidered. 



Hen birds possess only one 

 functional oviduct, the left ; 

 the chick before hatching has 

 two, but for some reason at 



FIG. 32. The cloaca of a Hen, 



showing the vestigial right ovi- present obscure, the right 



duct. C, Cloaca ; O, Oviduct. ^^ atroph j es> leaying at 



most a short tubular stump attached to the right 

 side of the cloaca (fig. 32). The right ovary is 

 either vestigial or altogether absent in birds. To the 

 stump of the right oviduct we may very justly apply 

 the term vestigial, and as will be demonstrated in the 

 chapter on tumours, such vestiges are by no means 

 devoid of danger, and even bring about the bird's 

 destruction. 



The abortion of the right oviduct in birds is in itself 

 very curious, especially when considered in connection 



