PAIRS OF CHARACTERS 135 



between 2 and 4, and the commonest number, based 

 on a large number of counts, is 3. 



It is only to the superficial observer that domi- 

 nance is complete in this instance, the first character 

 dealt with by Mendel. The microscope at once 

 reveals the fact that the pure and hybrid rounds, 

 though externally indistinguishable, are in fact of 

 radically different constitutions. This fact is also 

 revealed by a determination of the absorptive 

 capacities of the pure parental and hybrid rounds. 

 It will be remembered that the absorptive capacity 

 of the wrinkled pea was stated to be about 120, and 

 that of the pure round 80. The absorptive capacity 

 of the hybrid round is almost exactly intermediate 

 between the two. 



The hybrid round, therefore, is seen to resemble 

 the pure only in the fact of being round, and to differ 

 in what must be regarded as more fundamental 

 properties ; and it is probable that with further 

 inquiry the number of cases in which simple domi- 

 nance occurs, i.e. where the hybrid is identical with 

 the dominants, will be greatly reduced. 



The following considerations are suggested by 

 the facts set forth in this chapter. In the experi- 

 mental study of heredity, it is not sufficient to find 

 that round and wrinkled, or any other characters, 

 constitute a Mendelian pair, and are inherited in 

 Mendelian fashion. Some attempt must be made 

 to get behind the appearance of roundness, for 

 instance, and to find out what roundness really 

 consists in. If no such attempt is made the fact 



