Chromosomes and Genes 185 



species can be understood largely on the basis of a definite primary 

 set of rods and dots which can unite in different ways to form Vs. 

 Certainly in the course of phylogeny the intimate structure of these 

 chromosomal units must have changed considerably, and it may be 

 conceived (as it is by classic geneticists) that this intrachromosomal 

 genie change, perhaps along with dupHcations, deletions, and trans- 

 positions, suffices as an explanation of the evolution of these species 

 in spite of the morphological constancy of the chromosomal units. 

 The different rearrangements of the same basic arms would, then, be 

 without any genetic meaning. This might mean that the reshuffling 

 of the arms is due to chance or to some exigencies of the mitotic 

 mechanism, whatever the special mechanism may be in the individual 

 instances. It is rather difficult, at least to my way of thinking, to 

 dispense with such a typical and clear-cut process of cytological 

 evolution by considering it without meaning, a chance product, or, 

 at best, a means of keeping a group of genes together because of 

 some selective advantage. To work out the latter idea in detail 

 would, I think, lead to a hopeless welter of hypotheses ad hoc. I 

 realize, of course, that the classic theory of the gene requires either 

 some such explanation as that just hinted, or an assumption of chance 

 happenings without meaning. If, however, our way of looking at 

 the genie structure of the chromosome is accepted, this chromosomal 

 phylogeny may have a deep meaning in the sense of producing new 

 patterns of action of the entire chromosomes, involved in basic 

 processes of development, needed for evolution on the superspecific 

 level. It should be added that in these changes of the chromosome 

 sets some pericentric inversions and translocations also have been 

 involved (see Patterson and Stone, 1952). This does not affect the 

 general argument. 



There are a number of facts already available which one day may 

 turn out to be connected with chromosomal actions. Why are so many 

 homozygous translocations lethal which do not change anything in 

 genie content according to the classical theory of the gene? A position 

 effect, as an explanation, is difficult. It works well in mutant effects 

 of translocations (e.g.. Blond, Xasta in Drosophila). The same is true 

 of vitality changes and small generalized effects, as emphasized in 

 a previous chapter. But complete lethality as a typical effect seems 

 to be on a different level. Although the possibility of a homozygous 

 position effect for a lethal locus cannot be denied, it sounds rather 

 odd. A systematic quest for physiological differences between organ- 

 isms with normal chromosome sets and derived ones with translo- 



