242 Retrograde Varieties 



flowers, which are not liable to produce yellow 

 variations, as in Salvia, Aster, Centaurea, 

 Vinca, Polygala and many others. Even the 

 rare pale yellowish species of some of these 

 genera have no tendency in this direction. The 

 hvacinths are the most remarkable, if not the 

 sole known instance of a species having red and 

 blue and white and yellow varieties, but here the 

 yellow is not the bright golden color of the but- 

 tercups. 



The existence of varietal colors in allied spe- 

 cies obviously points to a common cause, and 

 this cause can be no other than the latency of 

 the pigment in the species that do not show it. 



The conception of latency of characters as the 

 common source of the origination of varieties, 

 either in the positive or in the negative way, 

 leads to some rules on variability, which are 

 known under the names given to them by Dar- 

 win. They are the rules of repeated, homolog- 

 ous, parallel and analogous variability. Each 

 of them is quite general, and may be recog- 

 nized in instances from the most widelv dis- 



« 



tant families. Each of them is quite evident 

 and easily understood on the principle of 

 latency. 



By the term of repeated variability is meant 

 the well-known phenomenon, that the same va- 

 riety has sprung at different times and in dif- 



