Biological Papers. 123 



use in accordance with the practice of a long line of ancestors. 

 Both body-building and body-using instincts are adaptive and 

 mutually reactive, a changing use inducing a changing struc- 

 ture, and a changing structure making possible a more rapidly 

 changing use. 



In the developing embryo life begins all the parts of 

 the body and completes several of them before the little 

 organism has any use for these structures. Thus the digestive 

 and respiratory apparatuses are ready for use before food and 

 air can enter them, and the blood-vessels and muscles are be- 

 gun before they are needed. So use by an ancestor determines 

 structure in a descendant, and not the operation of chance or 

 the effect of an external environment. Where use varies in the 

 parent, the structure and use vary in the descendant, but only 

 to an almost infinitesimal degree, fortunately, in all but a very 

 few of the descendants. 



Even a careless observer must have noted that the lower 

 animals possess body-building instincts and instincts proper 

 very much like our own. The evolutionist has inferred, be- 

 cause of this and other reasons, that the higher animals have 

 been evolved from the lower ones, not suddenly, but by the 

 accumulation of infinitesimal variations through long eras of 

 time, many millions of years in duration. 



It would not be germane to the purpose of this paper to at- 

 tempt to prove the correctness of this view of the evolutionist. 

 Indeed, so few now hold the contrary theory such an at- 

 tempt would scarcely be necessary under any circumstances. 

 It may be well, however, to mention one confirmatory proof 

 which has many interesting bearings on the subject we are 

 considering. 



Professor James says that certain instincts in man become 

 habits if the tendencies are used consciously in the perform- 

 ance of work; otherwise these instincts become dormant and 

 may never be fully functional during the lifetime of the indi- 

 vidual. In like manner, as has been already stated, the body- 

 building instincts become weakened in the descendant if the 

 part to be made had not been used by its ancestors ; and such 

 parts become vestigial in remote succeeding generations if the 

 disuse is persisted in. The eyes of the cave-fish have un- 

 doubtedly become vestigial in this way; and man's ancestors 

 have discontinued the use of so many parts that one anatomist, 

 Wiedersheim, declares that he has found 180 parts in man's 



