308 LAWS OF VAKIATION. Chap. XXIY. 



parts which, have becoine rudimentary under domestication. 

 We do not know through what steps under nature rudimentary 

 organs have passed in being reduced to their present condition ; 

 but we so incessantly see in species of the same group the 

 finest gradations between an organ in a rudimentary and 

 perfect state, that we are led to believe that the passage must 

 have been extremely gradual. It may be doubted whether a 

 change of structure so abrupt as the sudden loss of an organ 

 would ever be of service to a species in a state of nature ; for 

 the conditions to which all organisms are closely adapted 

 usually change very slowly. Even if an organ did suddenly 

 disappear in some one individual by an arrest of development, 

 intercrossing with the other individuals of the same species 

 would tend to cause its partial reappearance ; so that its 

 final reduction could only be effected by some other means. 

 The most probable view is, that a part which is now rudi- 

 mentary, was formerly, owing to changed habits of life, used 

 less and less, being at the same time reduced in size by 

 disuse, until at last it became quite useless and superfluous. 

 But as most parts or organs are not brought into action 

 during an early period of life, disuse or decreased action will 

 not lead to their reduction until the organism arrives at a 

 somewhat advanced age ; and from the principle of inheritance 

 at corresponding ages the reduction will be transmitted to 

 the offspring at the same advanced stage of growth. The 

 part or organ will thus retain its full size in the embryo, 

 as we know to be the case with most rudiments. As soon as 

 a part becomes useless, another princij^le, that of economy of 

 growth, wdll come into play, as it would be an advantage to 

 an organism exposed to severe competition to save the de- 

 velopment of any useless part; and individuals having the 

 |)art less developed will have a slight advantage over others. 

 But, as Mr. Mivart has justly remarked, as soon as a part is 

 much reduced, the saving from its further reduction will be 

 utterly insignificant ; so that this cannot be effected by 

 natural selection. This manifestly holds good if the part be 

 formed of mere cellular tissue, entailing little expenditure of 

 nutriment. How then can the further reduction of an already 

 somewhat reduced part be effected ? That this has occurred 



