CHROMOSOMES AND EVOLUTION 101 



number of spindle attachments of 10 to 16 in the 

 genus. The majority of species have one pair of 

 microchromosomes, but not all ; no species has more 

 than one pair of them. There is so far no means of 

 ascertaining whether the pair of microchromosomes 

 is always homologous throughout those species in 

 which it is found. 



It is worth while concentrating attention on two 

 groups of species, (1) that which includes D. melano- 

 gaster and D. simulans, (2) the obscura group, which 

 includes a large number of forms of which only D. 

 sub-obscura, D. miranda and the various races of 

 D. pseudo- obscura and D. affinis have been adequately 

 investigated. 



D. melanogaster and D. simulans have quite clearly 

 not evolved very far beyond the point at which one 

 or the other separated off as an incipient species. 

 The differences between their chromosomes have 

 already been described (page 73). The question 

 arises as to whether the original cause of species- 

 dichotomy was the inversion in the Ilird chromosome, 

 or whether this arose later, when the two forms were 

 already separated by the sterility-barrier in the Fj 

 hybrids. This question cannot be answered at 

 present. It is, however, extremely interesting that 

 the gene -differences between the two species (as 

 revealed by the sequence of ' bands ' in the salivary 

 gland chromosomes) are not distributed at random 

 along the chromosomes, but are concentrated in several 

 quite short regions. These regions are (1) the IV th 

 chromosome, (2) a region in the Ilird chromosome 

 between the inversion and the spindle attachment, 

 (3) the distal end of the X-chromosome. 



Now, although the frequency of mutation varies 

 from one gene to another, there is no reason to believe 

 that whole regions of the chromosomes are more 

 mutable than others (apart from the difference be- 

 tween active and inert regions). We are thus once 

 more driven to the same conclusion as we already 



