272 THE ORIGIN OF SPECIES 



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 retain 

 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 conse- 

 quence 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 addi- 

 tional explanation is here requisite which I cannot give. 

 If, for instance, it could be proved that every part of 

 the organization tends to vary in a greater degree toward 

 diminution than toward augmentation of size, then we 

 should be able to understand how an organ which has 

 become useless would be rendered, independently of the 

 effects of disuse, rudimentary and would at last be wholly 

 suppressed; for the variations toward diminished size 

 would no longer be checked by natural selection. The 

 principle of the economy of growth, explained in a 

 former chapter, by which the materials forming any part, 

 if not useful to the possessor, are saved as far as is pos- 

 sible, will perhaps come into play in rendering a useless 

 part rudimentary. But this principle will almost neces- 

 sarily be confined to the earlier stages of the process of 

 reduction; for we cannot suppose that a minute papilla, 

 for instance, representing in a male flower the pistil of the 

 female flower, and formed merely of cellular tissue, could 



