922 LIFE : OUTLINES OF GENERAL BIOLOGY 



whereas the horse standing on the street effects the same orientation 

 by moving its ear-pinnae and nothing more. 



We shall give no other examples from man's museum of relics, 

 but refer to Wiedersheim's Structure of Man (1895), where an almost 

 complete list, amounting to over a hundred, is given, the length of 

 the list being largely due to the inclusion of numerous vestigial 

 muscles, such as the traces of the skin-twitching panniculus 

 carnosus, which is well-developed in many mammals, such as the 

 horse. 



(c) Adult Cetaceans have no externally visible hind-limbs, yet 

 there are internal vestiges of several bones, with which muscles are 

 associated. In some cases there are relics of hip-girdle, femur, and 

 tibia, partly cartilaginous and partly bony. A large whale, thirty 

 feet long, may have a vestigial hip-girdle of a little over a foot. It 

 is not knov/n that the associated muscles are of any use ; the vestigial 

 pieces of skeleton are buried very deeply beneath the blubber ; and 

 the hip-girdle relic is not connected with the vertebral column. In 

 very young embrj^os of the porpoise and of a few other Cetaceans 

 there are actually external traces of the vanished hind-legs. 



{d) At the junction of the small and large intestine in many birds 

 there are two blind tubes or caeca forming culs-de-sac on the course 

 of the alimentary canal. In a duck, for instance, they are as long as 

 one's hand; and they have the usual function of caeca, that they 

 delay the food on its downward passage, affording longer opportunity 

 for digestion and absorption. But if we examine a pigeon instead of 

 a duck, we find two minute projections less than a quarter of an 

 inch in length — too small to be of any appreciable use. They illus- 

 trate clearly what is meant by vestigial structures, for every possible 

 gradation is found between the immense caeca of the ostrich and 

 their almost complete absence in an eagle. The reduction to a vanish- 

 ing-point may be correlated with the specialisation of diet and the 

 raising of the digestive function in birds to a very high level of 

 efficiency, as is indicated by the small amount of undigested residue, 

 the waste that accumulates in guano islands, for instance, being 

 mainly renal excretion. 



(e) In no snake is there any trace — even embryonic — of a pectoral 

 girdle and fore-limb, but there are occasional vestiges of a pelvic 

 girdle and hind-limb. Thus in the pythons there are two spurs about 

 half an inch long projecting from the under surface just in front of 

 the cloaca. As it is possible that these may be of use in the union 

 of the sexes, or even in climbing, we shall not insist on them; but 

 there are other cases where the total size is so minute that no one 

 can suggest utility. 



A recent study of the snake's vanishing hip-girdle has been made 

 by Prof. J. E. Duerden and Mr. R. Essex, of Grahamstown, the 

 subject being a black snake called Glaucoma. It is a burrowing 



