72 BASIS OF THE PEOBLEM 



contain both unit-factors. The hybrids, which, as we have seen, 

 exhibit the character A', will produce gametes, half of which 

 bear the unit-factor for A and half the unit-factor for a. When 

 the hybrids interbreed, on the average of chances one quarter of 

 the fertilized eggs will have two factors for A, one quarter two 

 factors for a, and one half both a factor for A' and a factor for 

 a. In this manner the splitting up of the second generation is 

 explained. 



What is important in this explanation is the conception of 

 unit-factors. The extension of the explanation to cover the more 

 complicated cases does not involve any modification of principle. 

 It follows that the germinal constitution contains a very large 

 number of unit-factors ; what therefore is innately given in the 

 germinal constitution is a collection of such unit-factors. Each 

 unit-character based upon a unit-factor can theoretically be 

 separately distinguished and isolated. The complications to 

 which references have been made are in part due to the difficulty 

 of distinguishing unit-characters. What is apparently a simple 

 character may not be a unit-character, but a combination of unit- 

 characters. This apparently simple character cannot appear 

 unless all the unit-factors, upon which these unit-characters are 

 based, are present in the germinal constitution. 



It may next be asked whether all unit-factors behave in this 

 manner when crossed. To this no definite .answer can yet be 

 given. The successful analysis of apparently contradictory cases 

 and the continual discovery of characters which do behave in 

 this fashion seem to point to an affirmative answer. The sugges- 

 tion is that, when this mode of behaviour cannot be demonstrated, 

 it is because the unit-characters have not yet been distinguished 

 and isolated. 



This brief reference to ' pure line ' investigations and to the 

 Mendelian analysis of crossing leads therefore to the following 

 conclusions. When, as in. the case of the self-fertilizing bean, 

 the strains are kept pure, existing germinal differences in respect 

 of any character are found to be of the nature of steps which are 

 usually small — though in outward manifestation the differences 

 are smoothed over by the influence of environmental stimuli. 

 This is the nature of the germinal differences, and they remain 

 what they are — apart from the origin of new factors or of the 

 loss of old factors, and apart from the effects of a differential 



