484 Genetic Theory and Evolution 



Babcock and others embrace the Neo-Darwinian doctrine as the only 

 way out, as they see it. 



It is possible that all these facts on the evolution of the karyotype 

 have no meaning at all, and are just chance happenings. When the 

 sauropsids originated (not considering such details as the aberrant 

 type of Crocodilia) the karyotype was there by chance and, not 

 having any significance, persisted even to the almost mammalian 

 Monotremata. But why did it change when the marsupials originated? 

 In the same way the different configurations in the drosophilids may 

 be the product of the chance alone that the first ancestor of each 

 species carried an otherwise meaningless translocation, and so on. It is 

 certainly possible to defend such a negative point of view, though it 

 will not appeal to many geneticists. The alternative is that these 

 features have a definite genetic meaning. What could this be? 



Within the classic theory of the gene a chromosome is a linkage 

 group, and a change in the karyotype means establishment of different 

 linkage groups. This might have an evolutionary meaning if by this 

 change groups of genes are kept together which for some reason are 

 needed as an intact group. In this case, however, inhibition of crossing 

 over would be needed. In addition, such an interpretation would have 

 a meaning only if genes concerned with similar or related functions 

 had an orderly distribution on the chromosome, which cannot be 

 asserted as a general fact. There is another possibility apparent. If 

 genes produce their primary products in loco and the first reactions 

 between them take place on the chromosomal surface, and if, further, 

 the reaction products diffuse along the surface of the chromosome to 

 enter into new reactions, all of which has been claimed, as we saw 

 before, the conclusion could be drawn that the establishment of new 

 linkage groups allows new primary reactions along the chromosome. 

 If such were the case, we should expect to find definite laws for the 

 change of the karyotype from species to genus to family, and so on, 

 and not the complete irregularity actually found. While it might be 

 very difficult to prove or even to make plausible such an interpretation, 

 it is equally difficult to disprove it, except by showing that the basic 

 assumptions regarding primary reactions are wrong. We discussed this 

 point previously and mentioned the obstacles to the acceptance of 

 this theory of genie action. 



If we renounce the classic gene and accept the theory of the 

 hierarchical pattern of the genie material, the changes in the karyo- 

 type would mean the introduction or removal of some of the cate- 

 gories of concerted action, especially the higher ones. But up to the 



