Chap. XXIV. EFFECTS OF USE AND DISUSE. 259 



be in part due to an inherent tendency, which is common to the females 

 of many Bombycidse, to have their wings in a more or less rudimentary 

 state ; but part of the effect may probably be attributed to long-continued 

 disuse. 



From the foregoing facts there can be no doubt that certain 

 parts of the skeleton in our anciently domesticated animals, 

 have been modified in length and weight by the effects of 

 decreased or increased use ; but they have not been modified, 

 as shown in the earlier chapters, in shape or structure. We 

 must, however, be cautious in extending this latter conclusion 

 to animals living a free life ; for these will occasionally be ex- 

 posed during successive generations to the severest competition. 

 With wild animals it would be an advantage in the struggle for 

 life that every superfluous and useless detail of structure should 

 be removed or absorbed ; and thus the reduced bones might 

 ultimately become changed in structure. With highly-fed do- 

 mesticated animals, on the other hand, there is no economy of 

 growth, nor any tendency to the elimination of trifling and 

 superfluous details of structure. 



Turning now to more general observations, Natkusius has 

 shown that, with the improved races of the pig, the shortened legs 

 and snout, the form of the articular condyles of the occiput, and 

 the position of the jaws with the upper canine teeth projecting in 

 a most anomalous manner in front of the lower canines, may 

 be attributed to these parts not having been fully exercised. For 

 the highly-cultivated races do not travel in search of food, nor 

 root up the ground with their ringed muzzles. These modifica- 

 tions of structure, which are all strictly inherited, characterise 

 several improved breeds, so that they cannot have been de- 

 rived from any single domestic or wild stock. 22 With respect 

 to cattle, Professor Tanner has remarked that the lungs and 

 liver in the improved breeds "are found to be considerably 

 " reduced in size when compared with those possessed by animals 

 " having perfect liberty;" 23 and the reduction of these organs 

 affects the general shape of the body. The cause of the reduced 

 lungs in highly-bred animals which take little exercise is ob- 



22 ISTathusius, 'Die Eacen des s. 103, 130, 333. 

 Schweines,' 1860, s. 53, 57; 'Vor- 23 ' Journal of Agriculture of Higli- 



studien .... Scliweiuescliadel,' 1864, laud Soc.,' July, 1860, p. 321. 



