310 University of California Publications in Agricultural Sciences [Vol. 2 



that while 53 per cent of the expected flies lacking one of the small 

 fourth chromosomes live, they are imperfect, weak, and often sterile. 

 That a small portion of a chromosome may be lost or inactivated is 

 indicated also by work on this fly (Bridges, 1919). Loss of this strain 

 is attributed to the injurious effect of the deficiency upon viability, 

 fertility, and productivity. 



While loss of chromosomes appears to be somewhat improbable as a 

 method by which one species can come to differ from another in chromo- 

 some number, the chromosome number of some species may be reduced 

 as a result of permanent end-to-end union of certain chromosomes to 

 form multiples. The differences in number noted for the Acrididae 

 (McClung, 1917) appear to be of this type. One species, Hesperotettix 

 viridis, shows considerable variation in chromosome union in different 

 individuals, indicating that it may be in the process of producing new 

 types of chromosome grouping. It is also decidedly variable morpho- 

 logically. 



There is some observational evidence that species differ from one 

 another in chromosome number due to cross-division of all chromosomes 

 of a complex. Marchal (1920), for example, reported that in the section 

 Medium of Campanula the size of each chromosome of pollen mother 

 cells is less when the haploid specific number is thirty-four than when 

 it is seventeen. 



It is difficult to understand how cross-division or union of chromo- 

 somes to form multiples could cause specific differences. In fact, a 

 case from Drosophila reported by Mrs. Morgan (1922) indicates that 

 while end-to-end union of the X-chromosomes may affect genetic 

 results it has no effect upon specifie characters. It seems simpler to 

 suppose that such changes in chromosome complexes are the result 

 rather than the cause of genetical differences between individuals, such 

 as have been noted for Hesperotettix viridis and for the different species 

 of the Acrididae. 



In the genu^ Drosophila, it has been shown that chromosomes that 

 look alike may carry very different genes. For example, in D. willistoni, 

 Metz and Lancefield (1922) report that the X-chromosome is a V- 

 shaped element similar to the second and third autosomes of D. melano- 

 gaster. Without this genetic evidence one would have said that these 

 two species had the same type of chromosome complex. Such evidence 

 is a timely warning to those who would draw hasty conclusions on the 

 basis of data like those given above for Crepis. The genetical results 

 from Crepis are still too scanty to permit of such tests. 



