476 BREAKDOWN OF GENETIC SYSTEMS 



regarded as representing the earliest and most unstable beginnings 

 of parthenogenesis yet found in plants. 



These mutations will also provide the basis of selection for 

 adaptation of the meiotic mechanism to more perfect conditions 

 for parthenogenesis, i.e., for developing the genetic character 

 which is the second step in the evolution of fertile parthenogenesis. 

 Nor should this be difficult. The appearance of genotypes for 

 the suppression or partial suppression of meiosis in mutants and 

 segregates of sexual species and hybrids is common enough 

 (Ch. X). And while in these any such mutation is a disadvantage, 

 in incipiently parthenogenetic organisms, it will have the highest 

 positive selection value. 



Conclusion. The following are the chief independent conditions 

 determining habitual apomixis : — 



(A) A genetic property of allowing the development of the egg-cell 

 without fertilisation and with or without other stimulus, such as 

 false fertilisation (Lidforss, 1914) or degeneration of neighbouring 

 tissues (Haberlandt, 1921, a and b). This is not inherent in all living 

 organisms, but is widespread in certain groups of plants and 

 animals, and is probably inherent in all those with normally zygotic 

 meiosis. 



(B) A genetic property determining the development of other 

 cells than the spore in the embryo-sac or embryo {cf. Andersson, 

 1 931) [condition of apospory and nucellar embryony] or of other 

 cells than the egg-cell into the embryo [condition of apogamy]. 

 This effect is usually conditional on failure of the facultatively 

 sexual embryo. 



(C) Functional hybridity, numerically or structurally incom- 

 patible with regular meiosis, leading to either (i) failure of numerical 

 reduction and partial suppression of segregation {cf. D., 1930 a) 

 which with (A) gives diploid parthenogenesis (cf. Harrison, 1920), 

 or (ii) sterility through segregation, whether in a mendelian sense or 

 as regards the distribution of whole chromosomes, which with (B) 

 gives apospory [cf. Chiarugi and Francini, 1930). 



(D) A genetic property determining : (i) fusion of a polar body 

 with the egg nucleus, or of two segmentation nuclei ; or (ii) failure 

 of numerical reduction [cf. P. Hertwig, 1920) owing to alteration of 



