CHROMOSOMES AND EVOLUTION 



107 



only rarely acts as the primary origin of a now 

 incipient species. 



Most so-called diploid organisms (such as the vast 

 majority of animal species) are really only partially 

 diploid and that part of their gene-complex which is 

 tetraploid is possibly less subject to the conserv^ative 

 effect of natural selection and is consequently in 

 more active evolution than the rest of the genes. In 

 bisexual animals, where polyploidy of whole chromo- 

 some sets is excluded by the sex-determining 

 mechanism the reduplication of small segments of 



TABLE X ^^' ^*' ®^' ®^' ^^^' ^^^ 



Diploid Chromosome Sets in the genus Drosophila ((^c?) 

 (V = a chromosome with median or submedian spindle 

 attachment, I = one with a quasiterminal attachment, m = 

 a microchromosome, A = the number of spindle attachments 

 on the assumption that V's have only one, B on the assumption 

 that they have two.) 



