Ohap. XXVI. SUMMARY. 353 



decreases the length of the intestines. Continued disuse, on 

 the other hand, weakens and diminishes all parts of the organi- 

 sation. Animals which during many generations have taken 

 but little exercise, have their lungs reduced in size, and as a 

 consequence the bony fabric of the chest, and the whole form of 

 the body, become modified. With our anciently domesticated 

 birds, the wings have been little used, and they are slightly 

 reduced; with their decrease, the crest of the sternum, the 

 scapulae, coracoids, and furcula, have all been reduced. 



With domesticated animals, the reduction of a part from 

 disuse is never carried so far that a mere rudiment is left, but 

 we have good reason to believe that this has often occurred 

 under nature. The cause of this difference probably is that with 

 domestic animals not only sufficient time has not been granted 

 for so profound a change, but that from not being exposed 

 to a severe struggle for life, the principle of the economy of 

 organisation does not come into action. On the contrary, we 

 sometimes see that structures which are rudimentary in the 

 parent-species become partially redeveloped in their domesticated 

 progeny. When rudiments are formed or left under domesti- 

 cation, they are the result of a sudden arrest of development, 

 and not of long-continued disuse with the absorption of all super- 

 fluous parts ; nevertheless they are of interest, as showing that 

 rudiments are the relics of organs once perfectly developed. 



Corporeal, periodical, and mental habits, though the latter 

 have been almost passed over in this work, become changed 

 under domestication, and the changes are often inherited. Such 

 changed habits in any organic being, especially when living a 

 free life, would often lead to the augmented or diminished use 

 of various organs, and consequently to their modification From 

 long-continued habit and more especially from the occasional 

 birth of individuals with a slightly different constitution, domestic 

 animals and cultivated plants become to a certain extent 

 acclimatised, or adapted to a climate different from that proper 

 to the parent-species. 



Through the principle of correlated variability when one part 

 varies other parts vary,-either simultaneously, or one after 

 the other. Thus an organ modified during an early embryonic 

 period affects other parts subsequently developed. When an 



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