The Method of Evolution 53 



dental loss of simple unit-characters, that the great color 

 variation of domesticated animals has arisen. 



We should naturally consider the color character of a 

 wild mouse, rat, or rabbit, to be very simple, for we observe 

 such animals to breed very true to color, but the behavior 

 of the wild type in crosses shows it in reality to be very com- 

 plex, and to be the result of the simultaneous presence 

 of some half-dozen or more wholly independent unit- 

 characters. New color varieties have arisen by loss or modifi- 

 cation of one or more of these unit-characters. For example, 

 the wild house mouse by simple loss of three independent 

 factors has given rise to seven additional varieties known 

 among fancy mice. 



The gray fur contains black, brown, and yellow pig- 

 ments disposed in a definite pattern in the individual hairs. 

 Loss of this pattern alone produces the black variety. Loss 

 of black produces the cinnamon variety; loss of both pro- 

 duces the bro\\Ti or chocolate variety. Loss of the power 

 to produce color, that is loss of some general color factor, 

 produces an albino, whose breeding capacity will varj- with 

 the number of other factors which it retains. Several other 

 color factors occur in mice, the loss of which has produced 

 new series of color varieties, but these will suffice to show 

 the process by which new varieties arise through loss of 

 unit-characters. 



Simple unit-characters are not confined to the super- 

 ficial parts of an animal, as for example to its fur. We 

 know these superficial characters best probably simph' 

 because they are most easily observed. 



Loss of horns in cattle behaves as a dominant unit- 

 character; likewise in man a shortened condition of the 

 skeleton producing two-jointed instead of three-jointed 



