184 Nature of the Genetic Material 



Here a simple explanation becomes visible. Systematic experiments 

 with the above-mentioned sections of homoeotic mutation might give 

 surprising results. This assumes that most of these mutants are 

 euchromatic, while we had concluded that podoptera is a heterochro- 

 matic effect. I wonder whether the detailed data of McClintock's work 

 on variegation in maize, when available, will supply such information. 



bb. The chromosome as a whole 



The last point of the argument should be, whether the hier- 

 archical order of the possible sections or fields of genetic action would 

 lead to its logical conclusion, eventual unified field action of an 

 entire chromosome. We have mentioned the work of Lima-de-Faria, 

 who demonstrated a visible chromosome structure involving a gra- 

 dient-like arrangement of the chromomeres which requires the pres- 

 ence of some organizing field covering an entire chromosome. It is 

 very difficult to imagine genetic facts that could prove the existence 

 of such a chromosomal field. Loss or addition of entire chromosomes 

 involves a change in genie balance, and observed effects may or may 

 not be entirely due to this upset. Cases in which actions of entire 

 chromosomes are suspected, as in sex determination, might be due 

 to such special features as heterochromatic action (see discussion in 

 1 2 C d ee) or simply to complete linkage of different loci. The so- 

 called Drosophila artificialis, with its reshuffling of entire chromosome 

 arms (Dubinin, 1936), would not be expected to be different, in 

 view of the well-known independence of arms in this genus. The 

 same might be true of plants like Datura in which exchanges of 

 complete, unitary arms seem to occur. The best hope lies with 

 pericentric inversions in chromosomes in which two arms right and 

 left of a centromere can be considered as independent chromosomes, 

 as in the Drosophila species. But it is not certain that a visible effect 

 of such rebuilt chromosomal arms, if found, must be attributed to 

 a chromosomal effect rather than to an ordinary position effect, as 

 already discussed for other translocations. Only if it were found 

 that all such inversions lead to one and the same type of large effect 

 (e.g., lethality in the egg stage or gross malformations) could a 

 decision in favor of chromosomal action be reached. 



A group of facts of indirect importance for this problem is found 

 in the comparative cytology of the species of Drosophila (excellent 

 review in White, 1945). Sturtevant and Novitski (1941) and their 

 followers (see details in Patterson and Stone's book, 1952) have 

 showTi that the configuration of the chromosome sets in Drosophila 



