412 CONCLUDING EEMAEKS. Chap. XXVIII. 



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variable than those distinguishing species, though hardly more 

 so than with certain protean species ; but this greater degree 

 of variability is not surprising, as varieties have generally been 

 exposed within recent times to fluctuating conditions of life, 

 are much more liable to have been crossed, and are still in 

 many cases undergoing, or have recently undergone, modifica- 

 tion by man's methodical or unconscious selection. 



Domestic varieties as a general rule certainly differ from each 

 other in less important parts of their organisation than do 

 species ; and when important differences occur, they are seldom 

 firmly fixed ; but this fact is intelligible if we consider man's 

 method of selection. In the living animal or plant he cannot 

 observe internal modifications in the more important organs ■ 

 nor does he regard them as long as they are compatible with 

 health and life. What does the breeder care about any slight 

 change in the molar teeth of his pigs, or for an additional 

 molar tooth in the dog; or for any change in the intestinal 

 canal or other internal organ? The breeder cares for the 

 flesh of his cattle being well marbled with fat, and for an 

 accumulation of fat within the abdomen of his sheep, and this 

 he has effected. What would the floriculturist care for any 

 change in the structure of the ovarium or of the oyules ? As 

 important internal organs are certainly liable to numerous 

 slight variations, and as these would probably be inherited, for 



many strange monstrosities are transmitted, man could un- 

 doubtedly effect a certain amount of change in these organs. 

 When he has produced any modification in an important part, 

 it has generally been unintentionally in correlation with some 

 other conspicuous part, as when he has given ridges and protu- 

 berances to the skulls of fowls, by attending to the form of the 

 comb, and in the case of the Polish fowl to the plume of feathers 

 on the head. By attending to the external form of the pouter- 

 pigeon, he has enormously increased the size of the oesophagus, 

 and has added to the number of the ribs, and given them 

 greater breadth. With the carrier-pigeon, by increasing, through 

 steady selection, the wattles on the upper mandible, he has 

 greatly modified the form of the lower mandible ; and so in many 

 other cases. Natural species, on the other hand, have been mo- 

 dified exclusively for their own good, to fit them for infinitely 



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