64 EVOLUTION 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 
Fic. 32.—The cloaca of a Hen, 
showing the vestigial right ovi- present obscure, the right 
duct. C, Cloaca; O, Oviduct. oviduct atrophies, leavileuea™ 
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 c 
