EVOLUTION OF MECHANISM 53 



be assumed that originally this modification of mitosis 

 applied to all mitoses in the organism. Later developments 

 would be restriction of meiosis in time and in space. 

 Ultimately, as living types evolved, it would be restricted 

 to certain events in gametogenesis, and the gametes would 

 come to be constitutionally haploid cells, and the diploid- 

 haploid mechanism would be perfected. 



Next, it may be assumed, came a differentiation of the 

 gametes to yield one kind that was fertilized and another 

 that fertilized. Then followed the development of homo- 

 hetero-gamety, one type of individual becoming so equipped 

 that perforce it must elaborate two kinds of gametes. This 

 probably was affected by development which reduced 

 crossing over. It is established that, in Drosophila at least, 

 there are genes which can and do reduce or even suppress 

 crossing-over. The suppression of crossing-over means that 

 qualitative differences can arise in the members of a chromo- 

 some pair, and that these cannot be transferred from one 

 member of the chromosome pair to the other, so that in 

 respect of these qualitative differences the individual 

 maintains a constitutionally heterozygous condition. It is 

 established, further, that reduction of crossing-over in the 

 case of one pair of chromosomes is attended by a similar 

 reduction of crossing-over in the case of all the rest of the 

 chromosomes within the chromosome complex. It may be 

 assumed that this suppression of crossing-over occurred in 

 the case of a pair of chromosomes which later were to 

 become the sex-chromosomes. As a result of this suppres- 

 sion, two chromosomes would evolve independently of one 

 another and would ultimately come to lose all qualitative 

 relationship. Following upon this would come quantitative 

 differences between the members of the sex-chromosome 

 pair. At first the members of this pair would be alike in 

 external structure. Part of one of them which, because of a 

 non-homology, could no longer pair with the corresponding 

 portion of the other, would become deleted so that there 

 would now remain an X-chromosome and a Y. The 

 Y-chromosome, by further deletion, would become smaller 

 and smaller, and finally the whole of it would be eliminated. 



