THEORIES OF HEREDITY 85 



its own far-off ancestry. Such cases have been made 

 familiar by the works of Darwin, who, for instance, men- 

 tions the zebra-like stripes on the fore-legs of horses, and 

 oftener still of mules ; or the reappearance of blue pigeons 

 resembling the wild rock-dove (Columbia livia) among a 

 breed of fancy pigeons : both being evident instances of 

 reversion to the old ancestral type of the respective species. 

 We can explain such reversions on the ground of Weis- 

 mann's theory of Determinants in the following manner : 

 We have already seen that the Ids of any species do not 

 all contain identical determinants representative of all the 

 racial characters, but that some of the determinants are 

 survivals of the old stages through which the species passed 

 during its racial evolution. Indeed, such old unmodified 

 determinants will be present in various numbers in nearly 

 every species, because to make a species stable in its charac- 

 teristics, it is only necessary that a majority of determinants 

 be moulded into the predominant form. A minority of 

 old determinants may well persist, and perchance, through 

 the means of reducing divisions and amphimixis in a series 

 of generations, by which each time germ-cells containing 

 such old determinants would be united, accumulate in the 

 germ-plasm, and thus ultimately come to expression in 

 an offspring. Even if a majority in number should not 

 always be attained, they might still be predominant, seeing 

 that the old determinants are all homodynamous, trying 

 to express the same character, while the other determinants, 

 if very distinct varieties are crossed, would be hetero- 

 dynamous in their action, and mutually cancel each other. 

 This explains why reversions are most likely to occur after 

 repeated crossings and recrossings with many varieties, 

 because it is then that the old racial determinants are most 

 easily able to assert their combined power against the 

 various heterodynamous new determinants of the many 

 crossed varieties. 



