The meaning of rudimentary organs is often quite unmistakeable:  for instance there are beetles of the same genus (and even of the same species) resembling each other most closely in all respects, one of which will have full-sized wings, and another mere rudiments of membrane; and here it is impossible to doubt, that the rudiments represent wings.  Rudimentary organs sometimes retain their potentiality, and are merely not developed:  this seems to be the case with the mammae of male mammals, for many instances are on record of these organs having become well developed in full-grown males, and having secreted milk.  So again there are normally four developed and two rudimentary teats in the udders of the genus Bos, but in our domestic cows the two sometimes become developed and give milk.  In individual plants of the same species the petals sometimes occur as mere rudiments, and sometimes in a well-developed state.  In plants with separated sexes, the male flowers often have a rudiment of a pistil; and Kolreuter found that by crossing such male plants with an hermaphrodite species, the rudiment of the pistil in the hybrid offspring was much increased in size; and this shows that the rudiment and the perfect pistil are essentially alike in nature.

An organ serving for two purposes, may become rudimentary or utterly aborted for one, even the more important purpose; and remain perfectly efficient for the other.  Thus in plants, the office of the pistil is to allow the pollen-tubes to reach the ovules protected in the ovarium at its base.  The pistil consists of a stigma supported on the style; but in some Compositae, the male florets, which of course cannot be fecundated, have a pistil, which is in a rudimentary state, for it is not crowned with a stigma; but the style remains well developed, and is clothed with hairs as in other compositae, for the purpose of brushing the pollen out of the surrounding anthers.  Again, an organ may become rudimentary for its proper purpose, and be used for a distinct object:  in certain fish the swim-bladder seems to be rudimentary for its proper function of giving buoyancy, but has become converted into a nascent breathing organ or lung.  Other similar instances could be given.

Rudimentary organs in the individuals of the same species are very liable to vary in degree of development and in other respects.  Moreover, in closely allied species, the degree to which the same organ has been rendered rudimentary occasionally differs much.  This latter fact is well exemplified in the state of the wings of the female moths in certain groups.  Rudimentary organs may be utterly aborted; and this implies, that we find in an animal or plant no trace of an organ, which analogy would lead us to expect to find, and which is occasionally found in monstrous individuals of the species.  Thus in the snapdragon (antirrhinum) we generally do not find a rudiment of a fifth stamen; but this may sometimes be seen.  In tracing the homologies of the same part in different members of a class, nothing is more common, or more necessary, than the use and discovery of rudiments.  This is well shown in the drawings given by Owen of the bones of the leg of the horse, ox, and rhinoceros.