n6 COMMON MODES OF INHERITANCE 



inheritance follows the parent whose germ-cells are the riper at 

 the time of fertilisation — an inference to which we shall return 

 in discussing germinal selection. 



The inference is further supported by Vernon's experiments in 

 the hybridisation of sea-urchins, for he showed that the characters 

 of the offspring incline to be those of the species whose gametes 

 were relatively the more mature when fertilisation occurred. 



§ 6. Alternative Inheritance 



Since the re-discovery of Mendel's Law — to which we shall 

 afterwards refer in detail — there has been a rapid accumulation of 

 instances of what is called alternative or Mendelian inheritance, 

 and some of the leading experimenters of to-day believe that this 

 mode of inheritance will be found to include other modes like 

 blended and particulate which seem at first sight distinct. 



Let us follow one of these authorities, Dr. C. B. Davenport, in 

 stating the fundamental ideas. 



Experimental work has driven home the conception of unit- 

 characters. That is to say, the characteristics of an organism 

 may be analysed in some cases into distinct units that are in- 

 herited independently. About a dozen of these, for instance, 

 have been demonstrated in the sweet-pea. 



The theory, supported by experimental results, is that these 

 unit-characters are represented in the germ-cells by what may 

 be called representative particles, or anticipatory units, or 

 primary constituents, or determiners, and that these cannot 

 blend or make any compromise with other determiners of con- 

 trasted unit characters. They are either there or not there. If 

 two parents have the same unit-character (x), the offspring get a 

 corresponding determiner x from both sides, and when the germ- 

 cells are formed in that offspring they will all have a double 

 determiner x, and will be like their parents as regards the unit- 

 character in question. If one parent has a unit-character (x) 



