Chap. XIIL 
EUDIMENTARY ORGANS. 
455 
further than by showing that rudiments can be pro¬ 
duced ; for I doubt whether species under nature ever 
undergo abrupt changes. I believe that disuse has been 
the main agency; that it has led in successive genera¬ 
tions to the gradual reduction of various organs, until 
they have become rudimentary,—as in the case of the 
eyes of animals inhabiting dark caverns, and of the 
wings of birds inhabiting oceanic islands, which have 
seldom been forced to take flight, and have ultimately 
lost the power of flying. Again, an organ useful under 
certain conditions, might become injurious under others, 
as with the wings of beetles living on small and exposed 
islands; and in this case natural selection would con¬ 
tinue slowly to reduce the organ, until it was rendered 
harmless and rudimentary. 
Any change in function, which can be effected by 
insensibly small steps, is within the power of natural 
selection; so that an organ rendered, during changed 
habits of life, useless or injurious for one purpose, 
might be modified and used for another purpose. Or 
an organ might be retained for one alone of its 
former functions. An organ, when rendered useless, 
may well be variable, for its variations cannot be 
checked by natural selection. At whatever period of 
life disuse or selection reduces an organ, and this will 
generally be when the being has come to maturity and 
to its full powers of action, the principle of inheritance 
at corresponding ages will reproduce the organ in its 
reduced state at the same age, and consequently will 
seldom affect or reduce it in the embryo. Thus we can 
understand the greater relative size of rudimentary 
organs in the embryo, and their lesser relative size in 
the adult. But if each step of the process of reduction 
were to be inherited, not at the corresponding age, but 
at an extremely early period of life (as we have good 
