OCR Text |
Show 450 RUDIMENTARY ORGANS. CHAP. XIII. vened at a very early age, or by. the variations havi.ng been inherited at an earlier period than that at whiCh they first appeared. It should also be born~ in mind, that the supposed law of resembl~nce of ancient forms of life to the embryonic stages of recent forms, may be true, but yet, owing to the geological reco~d not extending far enough back in time, may remain. for a long period, or for ever, incapable of de~onstrati~n. Thus, as it seems to me, the leading facts In embryology, which are seeond in impor~an?e to non.e in natural history, are explained on the pnnCiple of shght modi:fi. cations not appearing, in the many descendants from some one aneient progenitor, at a very early period in the life of each, though perhaps eaused at the earliest, and being inherited at a corresponding not early period. Embryology rises greatly in interest, when we thus look at the embryo as a picture, more or less obseured, of the common parent-form of each great class of animals. Rudimentary, atrophied, or aborted organs.-Organs or parts :in this strange eondition, bearing the stamp of inutility, are extremely common throughout natur~. For instance, rudimentary mammre are very general I~ th~ males of mammals : I presume that the "bastard-wmg in birds may be safely eonsidered as a digit in a rudimentary state: in very many snakes one lobe of th~ lungs is rudimentary; in other snakes there are rud1men~s of the pelvis and hind limbs. So~e of the ~ases of rudimentary organs are extremely cunous; for Instance, the presence of teeth in fcetal whales, which when grown up have not a tooth in their heads ; and th~ presence o; teeth, which never eut through the gums, In the uppe jaws of our unborn calves. It has even been stated on good authority that rudiments of teeth can be detected CHAP. XIII. RUDIMENTARY ORGANS. 451 in the .beaks of certain . embryonic birds. Nothing can be plainer ~han that wings ar~ formed for flight, yet in how many Insects do we see Wings so reduced in size as to be utterly incapable of flight, and not rarely lying under wing-cases, firmly soldered together! 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 mammre 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 welldeveloped state. In plants with separated sexes, the male flowers often have a rudiment of a pistil ; and Kolreuter found that by erossing such male plants with an hermaphrodite speeies, 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 rudi~ entary 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 eonsists of a stigma |