Chap. XXYIII. CONCLUDING EEMARKS. 407 



its nature appear to be the same in both. Varieties can be 

 classed in groups under groups, like species under genera, and 

 these under families and orders; and the classification may 

 be either artificial, — that is, founded on any arbitrary cha- 

 racter, — or natural. With varieties a natural classification 

 is certainly founded, and with species is apparently founded, 

 on community of descent, together with the amount of modi- 

 fication which the forms have undergone. The characters 

 by which domestic varieties differ from one another are more 

 variable than those distinguishing species, though hardly 

 more so than with certain polymorphic species; but this 

 greater degree of variability is not surprising, as varieties 

 have generally been exposed within recent times to fluctu- 

 ating conditions of life, and are much more liable to have 

 been crossed ; they are also in many cases still undergoing, 

 or have recently undergone, modification by man's methodical 

 or unconscious selection. 



Domestic varieties as a general rule certainly differ from 

 one another in less important parts 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 ad- 

 ditional 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 

 ovules ? As important internal organs are certainly liable 

 tc numerous slight variations, and as these would probably 

 be transmitted, for many strange monstrosities are inherited, 

 man could undoubtedly effect a certain amount of change in 

 these organs. When he has produced any modification in an 

 important part, he has generally done so unintentionally, 

 in correlation with some other conspicuous part. For in- 

 stance, he has given ridges and protuberances to the skulls 



