SUBSEXUALITY 477 



the time relationship of meiosis. With (A) this property gives 

 diploid parthenogenesis in many plants and animals. It may be 

 acquired as an adaptation by selection of segregates secondary to (C), 

 which alone is an imperfect basis for diploid parthenogenesis. 



(iii) Subsexual Reproduction, Apomixis has in the past been 

 investigated, described and classified in terms of morphology and 

 not in terms of genetics. This was at first inevitable since genetics 

 is founded on the study of sexually reproducing organisms, and the 

 rules it has established can be applied directly to them alone. Even 

 for them we need the help of chromosome studies when we are 

 dealing with hybrids of a complex character. These studies can be 

 used as we have seen by extrapolating from the relationships 

 between chromosome behaviour and breeding behaviour made out 

 in specially simpHfied experiments. Such studies we now see can 

 also be applied to the elucidation of apomixis from the genetical 

 point of view. 



The most obvious conclusions have already been drawn. They 

 show the part taken by mechanical relationships of the chromosomes 

 and by the adaptation of the genotype in the development of 

 apomixis. But there is a further step to be taken. Sexual repro- 

 duction has always, since Weismann, been seen to consist in the two 

 essential processes of meiosis and fertilisation. These processes are 

 the means of recombining the parts of chromosomes. Such a 

 recombination is possible, however, without either meiosis or 

 fertilisation. The process of segregation which is responsible for 

 the variation and development of parthenogenetic plants requires 

 merely crossing-over. And crossing-over is as we have seen a 

 universal concomitant of meiosis, an essential part of sexual repro- 

 duction, without which, in fact, sexual reproduction would become 

 effectively clonal for each chromosome. The retention of crossing- 

 over, even in the attenuated degree to which it is found in stable 

 apomictics, is therefore genetically, although not morphologically a 

 property of sexual reproduction. The parthenogenetic organism 

 with crossing-over has thus, as we may say, a suhsexual reproduction, 

 the sexual character of which is morphologically concealed, but 

 genetically effective, more effective indeed than in a regularly self- 

 fertilised sexual diploid. 



