262 KUDIMEXTAKY, ATROPHIED, [Chap. XIV. 



tions. Organs, originally formed by the aid of natural 

 selection, when rendered useless may well be variable, 

 for theu' variations can no longer be checked by natural 

 selection. All this agrees well with what we see under 

 nature. Moreover, at whatever period of life either disuse 

 or selection reduces an organ, and this will generally 

 be when the being has come to maturity and has to 

 exert its full powers of action, the principle of inherit- 

 ance at corresponding ages will tend to reproduce the 

 organ in its reduced state at the same mature age, but 

 will seldom affect it in the embryo. Thus we can 

 understand the greater size of rudimentary organs in 

 the embryo relatively to the adjoining parts, and their 

 lesser relative size in the adult. If, for instance, the 

 digit of an adult animal was used less and less during 

 many generations, owing to some change of habits, or if 

 an organ or gland was less and less functionally exercised, 

 we may infer that it would become reduced in size in the 

 adult descendants of this animal, but would retam nearly 

 its original standard of development in the embryo. 



There remains, however, this difficulty. After an 

 organ has ceased being used, and has become in con- 

 sequence much reduced, how can it be still further 

 reduced in size until the merest vestige is left; and 

 how can it be finally quite obliterated ? It is scarcely 

 possible that disuse can go on producing any further 

 effect after the organ has once been rendered functionless. 

 Some additional explanation is here requisite which I 

 cannot give. If, for instance, it could be proved that 

 every part of the organisation tends to vary in a greater 

 degree towards diminution than towards augmentation 

 of size, then we should be able to understand how an 

 organ which has become useless would be rendered, in- 



