346 LAWS OF VARIATION. Chap. XXVI. 



time sufficient for any very great change, and that the 

 principle of economy of growth does not come into action. 

 On the contrary, structures which are rudimentary in the 

 parent-species, sometimes become partially redeveloped in our 

 domesticated productions. Such rudiments as occasionally 

 make their appearance under domestication, seem always to 

 be the result of a sudden arrest of development ; 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 an 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 especi- 

 ally 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, taken in 

 its widest sense, when one part varies other parts vary, either 

 simultaneously, or one after the other. Thus, an organ modi- 

 fied during an early embryonic period affects other parts 

 subsequently developed. When an organ, such as the beak, 

 increases or decreases in length, adjoining or correlated parts, 

 as the tongue and the orifice of the nostrils, tend to vary in the 

 same manner. When the whole body increases or decreases 

 in size, various parts become modified ; thus, with pigeons 

 the ribs increase or decrease in number and breadth. Homo- 

 logous parts which are identical during their early develop- 

 ment and are exposed to similar conditions, tend to vary 

 in the same or in some connected manner, — as in the case of 

 the right and left sides of the body, and of the front and 

 hind limbs. So it is with the organs of sight and hearing ; 

 for instance, white cats with blue eyes are almost always 

 deaf. There is a manifest relation throughout the body be- 

 tween the skin and various dermal appendages, such as hair, 

 feathers, hoofs, horns, and teeth. In Paraguay, horses with 



