MUTUAL AFFINITIES OF ORGANIC BEINGS 409 



mentary organs is comparatively simple; and we can understand 

 to a large extent the laws governing their imperfect development. 

 We have plenty of cases of rudimentary organs in our domestic 

 productions, as the stump of a tail in tailless breeds, the vestige of 

 an ear in earless breeds of sheep — the re-appearance of minute 

 dangling horns in hornless breeds of cattle, more especially, ac- 

 cording to Youatt, in young animals — and the state of the whole 

 flower in the cauliflower. We often see rudiments of various parts 

 in monsters; but I doubt whether any of these cases throw light 

 on the origin of rudimentary organs in a state of nature, further 

 than by showing that rudiments can be produced; for the balance 

 of evidence clearly indicates that species under nature do not 

 undergo great and abrupt changes. But we learn from the study 

 of our domestic productions that the disuse of parts leads to their 

 reduced size; and that the result is inherited. 



It appears probable that disuse has been the main agent in ren- 

 dering organs rudimentary. It would at first lead by slow steps to 

 the more and more complete reduction of a part, until at last it 

 became rudimentary — as in the case of the eyes of animals in- 

 habiting dark caverns, and of the wings of birds inhabiting oceanic 

 islands, which have seldom been forced by beasts of pray 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 ex- 

 posed islands; and in this case natural selection will have aided 

 in reducing the organ, until it was rendered harmless and rudi- 

 mentary. 



Any change in structure and function, which can be effected by 

 small stages, is within the power of natural selection; so that an 

 organ rendered, through changed habits of life, useless or injurious 

 for one purpose, might be modified and used for another purpose. 

 An organ, might, also, be retained for one alone of its former 

 functions. Organs, originally formed by the aid of natural selec- 

 tion, when rendered useless may well be variable, for their varia- 

 tions 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 inheritance at cor- 

 responding ages will tend to reproduce the organ in its reduced 

 state at the same mature age, but will seldom affect it in the em- 

 bryo. Thus we can understand the greater size of rudimentary 

 organs in the embryo relatively to the adjoining parts, and their 



