2 52 GENERAL BIOLOGY 



in whole organisms has occurred most frequently as an 

 accompaniment of the parasitic manner of life, and 

 parasites, therefore, furnish its best illustrations. These 

 will be the subject of a special study in a succeeding chapter; 

 but retrograde development in the parts of organisms may 

 be noted here. An excellent example has already been 

 before us in the vestigial fifth stamen of Chelone (fig. 23r) : 

 and the figwort family to which this plant belongs, would 

 furnish a series of forms showing all grades of development 

 of this stamen from normal functional development to com- 

 plete atrophy. Most functionless organs that are found 

 larger and better developed and functional elsew^here, are 

 vestigial structures. They are developmental heirlooms; 

 useless and unnecessary, but handed down by heredity. 

 There is no other explanation for their existence. 



Often the disappearance of their function has accompanied 

 a change of habit or of habitat on the part of their possessor. 

 Thus the abundant stomates of the bladderwort, useless and 

 imperforate, although composed of the two guard cells as in 

 aerial plants, have doubtless been carried over from aerial into 

 aquatic life. The bladderwort (Utricularia) is descended 

 from terrestrial plants, but now lives wholly submerged 

 beneath the surface of the water. Many such shifts have 

 taken place in the phylogenetic development of the human 

 body. The greatest of them must have been from water to 

 land, from horizontal to erect attitude, and more latterly, 

 from savage to civilized conditions of living ; and the vestigial 

 structures are so numerous in man's body that it has been 

 called "a museum of antiquities." The best know^n of 

 these waning organs of vanished function are the vermi- 

 form appendix, the muscles for wagging the ears, the "wis- 

 dom teeth," and the hairy covering of the skin. Admir- 

 able examples of such organs are the rudimentary lung 

 and pelvic girdle of the snake. 



