686 LIFE : OUTLINES OF GENERAL BIOLOGY 



"Every function is the resultant of several components, of which 

 one is the chief or primary function, while the others are subsidiary 

 or secondary. The diminution of the chief function and the ascen- 

 dance of a secondary function changes the total function; the 

 secondary function becomes gradually the chief one; the result is 

 a transformation of the organ." The contraction of every muscle 

 is associated with an electrical change; the electric organ of the 

 Torpedo is a richly innervated mass of transformed muscular tissue 

 in which the production of electricity has become the main function. 

 The structure known as the allantois, which forms a foetal mem- 

 brane (chiefly respiratory) round the embryo of reptile and bird, 

 and a great part of the placenta of mammals, has for its homologue 

 an unimportant cloacal bladder in Amphibians. 



In this connection it should be noted that an organ may occa- 

 sionally change its function in the course of individud development. 

 The stalk of the embryo's allantois seems to become the adult's 

 urinary bladder. The thymus gland, which develops in connection 

 with the second embryonic gill-cleft or visceral cleft is largest in 

 the human infant just after birth, and becomes smaller and smaller 

 after the second year, until there is hardly a vestige left in adult life. 

 It appears to be an organ in which lymph corpuscles are produced in 

 the young animal, and then it dwindles. But an interesting fact is 

 that it persists in those mammals that hibernate, and that it there 

 changes its function, and becomes a fat-accumulating organ. 



RUDIMENTARY ORGANS.— This term is usually employed to 

 denote structures that have in the course of evolution dwindled 

 so markedly that they have lost all functional significance The 

 dwindling is inferred from the fact that they have well-developed 

 and functional homologues in related types. Thus the traces of hind- 

 limbs in whales are buried deep below the surface and are of no use ; 

 the third eyelid in man and monkeys is reduced to the small function- 

 less fold in the inner comer of the eye; one of the lungs, usually the 

 left, in snakes is represented by a small insignificant sac; the first 

 gill (pseudobranch) of a Teleostean or bony fish is usually a small 

 vascular patch, utterly unimportant. Other illustrations are given 

 in some detail among the Evidences of Evolution. The best term 

 for these structures is vestigial; for "rudimentary" is often (and 

 better) used to mean incipient, and it is essential to the definition 

 of the structures we are now discussing that they can be shown to 

 be the useless remnants of structures that were once well-developed 

 and functional. The electric organ on each side of the tail of a skate 

 may be, in a large specimen, two feet long and an inch across, but 

 it does not produce a shock strong enough to be felt by the human 

 hand. But as its structure is not in any way degenerate, and as it 

 is one of the last organs to be established in the course of develop- 



