88 ON HEREDITY. 



the growing organ, which is favoured by natural selection in its 

 increased blood-supply, etc. ; while the operation of natural selection 

 has also determined the organ which can bear a corresponding loss 

 without detriment to the organism as a whole. 



Without the operation of natural selection upon different indi- 

 viduals, the struggle between the organs of a single individual 

 would be unable to encourage a predisposition in the germ towards 

 the degeneration or non-development of a useless organ, and it 

 could only limit and degrade the development of an organ in the 

 lifetime of the individual, j If, therefore, acquired characters are not 

 transmitted, the disposition to develope such an oigan would be 

 present in the same degree in each successive generation, although 

 the realization would be less perfect. The complete disappearance 

 of a rudimentary organ can only take place by the operation of 

 natural selection ; this principle will lead to its elimination, inas- 

 much as the disappearing structure takes the place and the nutriment 

 of other useful and important organs. Hence the process of natural 

 selection tends to entirely remove the former. The predisposition 

 towards a weaker development of the organ is thus advantageous, 

 and there is every reason for the belief that the advantages would 

 continue to be gained, and that therefore the processes of natural 

 selection would remain in operation, until the germ had entirely 

 lost all tendency towards the development of the organ in question. 

 The extreme slowness with which this process takes place, and the 

 extraordinary persistence of rudimentary organs, at any rate in 

 the embryo, together with their gradual but finally complete dis- 

 appearance, can be clearly seen in the limbs of certain vertebrates 

 and arthropods. The blind-worms have no limbs, but a rudi- 

 mentary shoulder-girdle is present close under the skin, and the 

 interesting fact has been quite recently established l that the fore- 

 limbs are present in the embryo in the form of short stumps, which 

 entirely disappear at a later stage. In most snakes all traces of 

 limbs have been lost in the adult, but we do not yet know for 

 certain whether they are also wanting in the embryo. I might 

 further mention the very different stages of degeneration witnessed 

 in the limbs of various salamanders ; and the anterior limbs of 

 Hesperornis the remarkable toothed bird from the cretaceous rocks 



1 Compare Born in 'Zoolog. Anzeiger,' 1883, No. 150, p. 537. 



