The progress of Genetics since the rediscovery of Mendel's papers. 401 



these Fl seeds; for some give round-seeded plants, others give indent, 

 in a normal Mendelian way. Plainly therefore the mother-plant 

 exercises some control over the shapes of the seeds. It is to be 

 remembered that in the cross indent X wrinkled, segregation is 

 normal, and the same is true of the cotyledon-colours if a green 

 variety is used. 



How this control is effected is quite obscure. At least three 

 other similar cases are known. In Wheat, Biff en (12) showed that 

 the seeds of F^ between a long-seeded polonicum wheat and a short- 

 seeded wheat show no mixture of type. All are long-intermediate 

 though segregation is proved by the next generation to have occurred. 

 He found the same true of the characters "glutenous" and "starchy". 

 Lock (71), working with Zea Mays observed the same phenomenon 

 regarding the characters "dent" and "flint", no mixture of type 

 occurring on the single plants. 



The very natural suggestion has been made that the seed-coat 

 is the determining cause, but the case of wrinkled X indent in Fisum 

 makes this suggestion difficult to apply. 



The Inheritance of Sex. 



The outcome of Mendelian analysis goes to show that the ex- 

 istence of diversity among a family of sister-organisms is due to the 

 existence of polymorphism among the gametes. Such a case of 

 diversity among members of a family exists in the phenomenon of 

 sexual dimorphism, and the question at once arises whether sex may 

 not be a consequence of gametic segregation (R. E.G. (10) p. 138). It 

 is interesting to know that this question occurred to Mendel him- 

 self, and that he instituted experiments to investigate it (see MendeVs 

 Briefe^ p. 241, Correns (40)). This question has been discussed in 

 some detail in a valuable essay by Castle (14, a), who showed that 

 it could be supported by several observ^ed facts of sex-production. 

 More recently Don caster (52) has put forward an ingenious hypo- 

 thesis on similar lines, showing how the important observations made 

 by himself and Ray nor on the inheritance of a colour- variation in 

 the moth Abraxas grossulariata can be reduced to a simple Mendelian 

 scheme. The variation in question, is a partial albino form, known 

 as lacticolor. Speaking of the type as D and the recessive as R, 

 experiment showed that sex was distributed among the offspring 

 according to a definite plan thus: 



26* 



