126 LECTURES ON BIOLOGY 



its most eloquent expression in heredity, such parts of the body 

 reappear more or less distinctly during the embryonic stage, 

 long after they have been lost in the adult animal. We have 

 already considered the evolution of the human circulatory system ; 

 further instances are that the human embryo, like that of an 

 anthropoid Ape, possesses thirteen pairs of ribs, whilst adult man 

 has only twelve ; that at an early stage the coccyx is compara- 

 tively large and protrudes beyond the lower extremities ; and that 

 in the carpus of the embryo an Os cent rale is rudimentary, just 

 as in amphibians and reptiles. In view of these and many other 

 facts that can be adduced every unbiassed student will admit 

 that the Evolution Theory is proved by abundant evidence, and 

 applies to the animal kingdom as well as to the human species. 



In adult man we are able to demonstrate the presence of 

 numerous organs and parts of organs which are either quite 

 rudimentary and useless, or in an obvious state of degeneration. 

 Like the males of all mammals, Man possesses rudimentary 

 teats. There are even certain cases known in which milk was 

 secreted by the male mammary gland. On the eyes of birds, 

 and in many fishes, amphibians, and reptiles is found a thin skin, 

 the so-called nictitating membrane or third eyelid. In the lower 

 mammals this membrane is still fully developed, but in the 

 higher mammals and in man it has been reduced to a tiny 

 rudiment, a crescent-shaped fold which lies in the inner angle 

 close to the lachrymal gland. 



The muscles of the ear are apparently in a similar state of 

 retrogressive development. True, there are still the same muscles 

 as in mammals, the attollens, attrahens, and retrahens aurem, 

 but they are only faintly developed and generally without 

 functions. This is so much true that in a person who is still able 

 to move his auricle this atavistic ability is regarded as an amusing 

 freak. 



Other rudimentary organs are the wisdom tooth, which in the 

 European is sometimes entirely absent ; the coracoid process of 

 the scapula, a bone which is strongly developed in reptiles, birds, 

 and the lowest mammals, the Ornithorhynchus, or duck-mole, 

 but rudimentary in man ; and, finally, the greatly degenerate tail- 

 muscles. 



The best known of all rudimentary organs is doubtless the 



