TAXONOMY, ANATOMY, EMBRYOLOGY 



brane. In most vertebrates, this "third eyehd" can be swept clear across 

 the eyeball to cleanse the latter, much like the blinking of a mammal. In 

 birds, it is particularly well developed. Its use can be easily observed if 

 a captive owl is watched by daylight. But in mammals, it forms a mere 

 crescentic fold at the inner corner of each eye, with no known— or prob- 

 able—function. It is understandable only as a degenerating inheritance 

 from an ancestor to which the nictitating membrane was actually useful, 

 as it now is to the majority of vertebrates. The ear muscles of man present 

 a similar situation. Many mammals move the external ear freely in order 

 to detect sounds efficiently. The complete muscular apparatus for these 

 movements is present in man, but quite vestigial. While school boys some- 

 times pride themselves on their ability to wiggle their ears, the ability has 

 no real usefulness, and even this limited abihty is not shared by everyone. 

 The presence of these muscles, then, suggests descent from an ancestor to 

 which they were really useful. 



Similarly, most mammals have a well-developed tail, but this is lacking 

 in all of the higher Primates. It is represented in them by vestigial caudal 

 vertebrae, usually three to five in man. Rarely, a fleshy tail extends a few 

 inches beyond the caudal vertebrae. Whether or not an external tail is 

 present, the muscles which move the tails of other mammals are also 

 present in the Primates. 



A final example from man concerns the wisdom teeth. The wisdom 

 teeth, or third molars, are the posterior-most teeth, as well as the last to 

 erupt. In other Primates, these teeth are as sound and as fully developed 

 as the rest of the dentition. But in man, they are far more variable than 

 are the other teeth with respect to size and time of eruption. Frequently, 

 they fail to erupt altogether. And when they are present, they are far more 

 subject to all types of dental defect than are the other teeth. Thus it is 

 probable that these teeth should be regarded as vestigial, and, in view of 

 the frequency with which they fail to erupt, that they will in time be 

 completely lost to man. 



Many examples of vestigial characters may be found among lower ani- 

 mals. The external ears of whales are completely of the type found in 

 terrestrial mammals, but they are much reduced in size, and it seems 

 unlikely that they are efficient auditory organs. Also among whales, the 

 hind limbs are completely missing, yet in some species rudiments of the 

 pelvic girdle still remain, but have lost their connection to the vertebral 

 column. In ungulates (horse, deer, and other hoofed animals), the smaller 

 bone of the lower rear leg, the fibula, has been reduced to a mere splint 

 on the larger bone, the tibia. A similar reduction of the fibula has occurred 

 in the birds. Perhaps no feature of the anatomy of snakes is so generallv 

 known as their leglessness. No snake shows any vestige of the forelimbs, 

 but some (pythons and boas) have small, ineffective rudiments of the 

 hindlimbs. (See Figure 96, p. 251.) These are capped by claws which 

 show externally, but they are so reduced that they appear at a glance to 

 be scarcely more than raised scales. 



Many animals, both vertebrates and invertebrates, have become adapted 

 to life in deep caves, where the light of the sun never reaches. Living 



43 



