STERILITY AND APOMIXIS 471 



ground the shortcomings of the notions of hybridity current amongst 

 geneticists. There is no use in considering the effect of hybridity 

 alone on sexual fertility because there is a group of conditions 

 having the same effect on fertility as the structural hybridity 

 characteristic of interspecific crosses, and this group of conditions 

 must be considered with it. They are the numerical hybridity of 

 triploids and pentaploids and the autopolyploidy which leads to 

 multivalent formation and consequent irregular segregation even 

 in tetraploids. The type of organisms in which the relationships 

 of the chromosomes determine infertility, i.e., hybrids and auto- 

 polyploids, must always be considered as a group in contrast with 

 those in which irregular or suppressed meiosis is physiologically 

 and not mechanically controlled. 



In a hybrid their dissimilarities prevent the chromosomes coming 

 together at pachytene, although we are not usually able to show 

 this cytologically except by inference from the later behaviour. 

 Now this property depends entirely on the relationship of the two 

 chromosomes. But precisely the same result can be obtained, not 

 through dissimilarity, but through the hereditary properties of the 

 individual. In Rhahditis pairing of chromosomes fails, although 

 they are known to be identical, and this has been shown to be 

 genotypically controlled, i.e., a property of the germ-plasm as a 

 whole, and not of the relationships of its parts. 



Viewed in this light, all apomixis can be seen to be primarily 

 conditioned by the hereditary properties of the individual which 

 makes development possible without fertilisation. Further, it can 

 be seen that the development of this hereditary property is in miany 

 cases independent of any structurally controlled failure of meiosis. 

 This is most obvious : (i) in organisms that are cyclically or faculta- 

 tively parthenogenetic (Aphides, Rotifera, Cladocera, Apotettix 

 and Paratettix, Allium, etc.) ; (ii) in others in which parthenogenesis 

 is a means of sex determination (Hymenoptera, Rotifera) ; (iii) in 

 many pseudogamous plants with good pollen [Zephyranthes, 

 Potentilla) ; and (iv) in animals and plants in which the pairing of 

 the chromosomes is perfect [Rhahditis, Solenohia triquetrella), 

 especially those which are facultatively sexual and true breeding 

 (Elatostema sessile, Thalictrum piirpiirascens, Allium odorum) 



