30 THE FACTORS OP ORGANIC EVOLUTION. 



wild duck and the domestic duck, &quot; the great and inher 

 ited development of the udders in cows and goats,&quot; and 

 the drooping ears of various domestic animals. Here are 

 other passages taken from the latest edition of the work. 



&quot; I think there can be no doubt that use in our domestic animals has 

 strengthened and enlarged certain parts, and disuse diminished them ; and 

 that such modifications are inherited &quot; (p. 108). [And on the following 

 pages he gives five further examples of such effects.] &quot; Habit in producing 

 constitutional peculiarities and use in strengthening and disuse in weaken 

 ing and diminishing organs, appear in many cases to have been potent in 

 their effects &quot; (p. 131). * When discussing special cases, Mr. Mivart passes 

 over the effects of the increased use and disuse of parts, which I have 

 always maintained to be highly important, and have treated in my Varia 

 tion under Domestication at greater length than, as I believe, any other 

 writer &quot; (p. 176). &quot; Disuse, on the other hand, will account for the less 

 developed condition of the whole inferior half of the body, including the 

 lateral fins &quot; (p. 188). &quot;1 may give another instance of a structure which 

 apparently owes its origin exclusively to use or habit &quot; (p. 188). &quot; It 

 appears probable that disuse has been the main agent in rendering organs 

 rudimentary &quot; (pp. 400 401). &quot; On the whole, we may conclude that habit, 

 or use and disuse, have, in some cases, played a considerable part in the 

 modification of the constitution and structure ; but that the effects have 

 often been largely combined with, and sometimes overmastered by, the 

 natural selection of innate variations &quot; (p. 114). 



In his subsequent work, The Variation of Animals and 

 Plants under Domestication, where he goes into full detail, 

 Mr. Darwin gives more numerous illustrations of the 

 inherited effects of use and disuse. The following are some 

 of the cases, quoted from volume i of the first edition. 



Treating of domesticated rabbits, he says: &quot;the want of exercise has 

 apparently modified the proportional length of the limbs in comparison with 

 the body &quot; (p. 116). &quot; We thus see that the most important and complicated 

 organ [the brain] in the whole organization is subject to the law of decrease 

 in size from disuse &quot; (p. 129). He remarks that in birds of the oceanic 

 islands &quot; not persecuted by any enemies, the reduction of their wings has 

 probably been caused by gradual disuse.&quot; After comparing one of these, the 

 water-hen of Tristan d Acunha, with the European water-hen, and showing 

 that all the bones concerned in flight are smaller, he adds &quot; Hence in the 

 skeleton of this natural species nearly the same changes have occurred, only 

 carried a little further, as with our domestic ducks, and in this latter case I 

 presume no one will dispute that they have resulted from the lessened use of 

 the wings and the increased use of the legs &quot; (pp. 286-7). &quot;As with other 

 long-domesticated animals, the instincts of the silk-moth have suffered. The 



