Evolution of the Genie Material 485 



point of discussion reached so far, not much is gained by this inter- 

 pretation, because it would also lead to the postulate of some order in 

 the observed facts of phylogeny of the karyotype. However, the 

 situation is different when we proceed from the karyotype to the 

 structural pattern of the chromosome itself. Here the salivary chromo- 

 somes of Drosophila give very pertinent information, most of it owing 

 to Dobzhansky and students (see Dobzhansky's book, 1952) and the 

 Patterson group (see Patterson and Stone, 1952). In species crosses, as 

 far as they succeed, it appears that more or less extensive parts of the 

 salivary chromosomes are normally synapsed and therefore are struc- 

 turally and genetically identical. In other regions smaller or larger 

 inversions and sometimes translocations can be traced; still other 

 regions do not synapse at all and seem structurally completely dif- 

 ferent. Though only a few species can be crossed successfully (see 

 Patterson and Stone, 1952), it is probable that the dissimilarity of the 

 structural pattern increases with decreasing taxonomic relation. The 

 patterns of these chromosomes between (uncrossable) species are so 

 different that they cannot be compared directly, and if we take 

 different genera, or even families of Diptera, the comparability of 

 pattern ceases to exist (e.g., Drosophila— > Sciara-^ Chironomus). 

 Whatever the details, at least one definite fact is visible, namely, 

 that the intimate structural pattern of the chromosomes changes, 

 diversifies with taxonomic distance. There is no reason to deny that 

 this is a general feature, though only the giant salivary chromosomes 

 of Diptera permit observing it. 



How can this structural diversification, which must have taken 

 place with evolution, be understood? I cannot see how the classic 

 theory of the gene could answer this question. The only answer it has 

 for the evolution of the genie material in the chromosomes is found in 

 the explanation of the origin of new genes : if a protozoan chromosome 

 is organized like any other one, this is only a matter of externa. The 

 decisive difference is that it has fewer genes, which, then, means that 

 evolution, beyond the reshuffling of mutant genes, consists of the 

 addition of new genes at each step. As a spontaneous generation of 

 genes is improbable, the new genes must be derived from old ones. 

 Bridges introduced the idea that a duplicated gene, first in tandem 

 duplication, later located anywhere else by translocation, may in time 

 transform into a completely new gene. The presence of the old one 

 permits such a development of a new one without harm to the 

 organism. 



I have always felt that this idea is very crude and, in addition, 



