l86 DAVENPORT 



characteristics which will cover all these cases? I think we 

 can and that it may be called the law of potency. At the one 

 extreme of the series we have equipotent unit characters, so that 

 when they are crossed the offspring show a blend, or a mosaic 

 between them. This is nearly the case with the length of ear 

 in rabbits, as Castle has found ; it is probably also true for 

 length of beak in poultry. At the other extreme is allelo- 

 potency. One of the two characteristics is completely recessive 

 to the other. This allelopotency is sometimes uniform, as in 

 the case of taillessness in No. 117, heretofore referred to; or 

 in some strawberry hybrids. Usually, it is alternating like tail- 

 lessness in the son of No. 117. Typical Mendelism comes 

 under the head of alternating allelopotency and it is to Morgan 

 that we owe the first, clear appreciation of the fact. According 

 to his theory, purity of the germ cells does not exist ; but a 

 dominant characteristic involves a recessive, as, in electricity, 

 a positive charge involves a negative one. Similarly a recessive 

 characteristic always carries hidden a dominant one. In the 

 first hybrid generation the dominant character is alone evident, 

 but in that generation two kinds of germ cells are produced 

 with opposite characteristics latent. Thus the dominant germ 

 cells have the recessive character latent; the recessive germ 

 cells have the dominant character latent. And segregation is 

 rather functional than structural. 



Between the two extremes of equipotency and allelopotency 

 lies the great mass of heritable characteristics which, when 

 opposed in heredity, exhibit var^ang degrees of potenc3^ This 

 sort of inheritance may be called heteropotency. Heteropotency 

 is exhibited in the case of the elements that make up the Y-comb 

 and in extra toes. It is the usual form of inheritance — that 

 which the breeder relies on when he selects the best appearing 

 animal or plant ; or that which throws the largest percentage of 

 favorable offspring. 



The law of potency seems thus as a general expression for 

 all the forms and exceptional cases met with in heredity. But 

 it does little to help us toward an understanding of the physical 

 basis of heredity, or towards its control. The next step is to 

 determine the conditions of the different sorts of potency. We 



