66 HEREDITARY CHARACTERS 



always disappears when the selection which has produced it 

 ceases. The example of the sugar produced from beetroot 

 is cited. The percentage of sugar in beetroot has been raised 

 by artificial selection from 7 per cent, or 8 per cent, to 15 per 

 cent., but directly selection ceases the percentage of sugar 

 returns to what it was originally. 



This fact, although of great interest, may not, and very 

 likely does not, mean anything more than that characters 

 rapidly produced by selection will disappear with equal or 

 greater rapidity when the selection ceases ; particularly in 

 the case of characters produced by the selection of small 

 variations. In this case of the beetroot, selection can at 

 most have been in action for but a few years, and the pro- 

 bability is that such a character is stable more or less in 

 proportion to the length of time for which selection has been 

 acting upon it. 1 We should therefore naturally expect it to 

 disappear with extreme rapidity upon the cessation of the 

 selection. 



We know that the disappearance of characters no longer 

 acted upon by selection is almost as common as the produc- 

 tion of new characters by the same process. In organisms 

 that have not been subject to domestication, characters are, 

 however, much more stable and take a very long time to 

 disappear. An example of a structure in process of dis- 

 appearing is the vermiform appendix in man. Extreme 

 examples of the disappearance of characters due to the 

 cessation of selection may be found among parasitic animals. 

 We may take as an instance some of the parasitic Crustacea 

 which have relations who, not having taken to a parasitic life, 

 have retained those characters useful to them in leading an 

 independent existence. Cyclops is one of the free-swimming 

 Eucopepoda found everywhere in fresh and brackish water. 



1 I have been unable to ascertain whether or not the environment was main- 

 tained in the case of the rapid fall in the percentage of sugar yielded by the 

 beetroot. If it was not, there is of course nothing in the change of percentage of 

 sugar, which may not have been a modification due to change of environment. 

 See description of Nageli's experiments with alpine plants, p. 136. 



