SKELETONS OF YOUTsTi ORGANS IN ADULTS. 105 



and developed ah initio, instead of being built up 

 upon those first formed. Such complete change, 

 however, necessitates a state of existence during which 

 action or function remains in complete abeyance. In 

 the pupa or chrysalis period of life, functional activity 

 is reduced to a minimum, and nothing is allowed to 

 interfere with the developmental and formative pro- 

 cesses. The new and more perfect being which is 

 evolved does not probably retain a trace of the struc- 

 ture of its earlier and less perfect state. Although 

 the elements of matter in the imago are, of 

 course, those of which the larva was composed, 

 they have been as completely re-aiTanged as they 

 would have been had they been introduced into the 

 organism of another individual altogether. Not 

 only have the old tissues been utterly destroyed 

 and new ones produced, but in many instances these 

 new ones belong to a totally different type ; and were 

 it not that observation has taught us that they have 

 been really evolved at different periods during the 

 life of one and the self-same individual being, we 

 should have concluded not only that they belonged to 

 different species, but in many cases to species far 

 removed from one another. 



153. Skeletons of young origans in adults. — In ver- 

 tebrate animals there is not an organ in the adult but 

 retains, not only the form which it assumed at a com- 

 paratively early period, but some of the very same 

 tissue which was active in early life remains in an 

 altered but deteriorated state. Every adult organ 

 may be said to contain as it were the imperfect 

 skeletons of organs which were active at an earlier 

 period of life. This material which slowly accumu- 

 lates, clogs, and perhaps even in the most perfect 

 state of things, slightly interferes with the free 

 activity of the organ. If from any interference with 

 the changes this unabsorbed debris accumulates in 

 undue proportion, the action of the organ may be 



