DEPARTMENT OF EXPERIMENTAL EVOLUTION. 113 



COMPARATIVE STUDIES ON THE GERM-PLASM OF DROSOPHILA. 



Dr. Metz initiated two or three j'-ears ago a series of breeding experi- 

 ments on other species of Drosophila to provide for a detailed genetic 

 comparison with the results ^ 



on Drosophila ampelophila. ^\ , ^s vv ff^ v\ '/ \\ ^ 

 Such a comparison involves ^x * \\ •/ ^**^ y/ I ^ J J ^ 

 numerous questions of wide )|^ \\ ^ lie II 



genetic and evolutionary 



significance, as well as sev- ^^ ^ ^ 0i \^ '/ li it 



eral of more technical in- yv : ^ ^ : ^ ^ ^ •i^:^^ 

 terest. On some of these ii 11^ II ^^ 



questions definite evidence "^ '''' " ^^ " 



has already been obtained, ^ /j ^ ^/ 61 11 



chiefly by the breeding of^i^^:^ j ^ » 



Drosophila virilis, although ^^\ /M ^.^^** *^ ' ^ 



theworkhasnot yet reached ^N? ' «^fM y^t j j'(\\ 



its most important stages. ^ 



Among the results attained ^O *^ ^^ ^f 



by Dr. Metz largely or ^ 2 ^ ^ 5 ^ \^ T^ 5) (f 

 wholly within the last year, ^^ ^ ^^| Vn 01 



he considers the following as ^^ ? «. •• 



the most important: ^' 



WiY/i regard to the relative mutability of species. — On this point the 

 evidence from virilis indicates a degree of mutability at least equal to 

 that in ainpelophila. Hence the supposed phenomenal mutability of 

 the latter is not limited to one species alone. At least 23 mutant races 

 have been obtained in D. virilis up to date. Considering the rela- 

 tively limited number of individuals examined, this undoubtedly repre- 

 sents a rate of mutation approximately equal to that of ampelophila. 



With respect to the nature of mutations arisi7ig in related species. — On 

 this point the evidence agrees with expectation in that the mutants in 

 virilis are of the same general types as in ampelophila; i. e., they repre- 

 sent modifications in the color, shape, and structure of the eyes, in the 

 color and shape of the body, in the size, shape, and venation of the 

 wings, in the number and morphological character of the bristles, etc. 

 Some of these modifications show strong indications of actually being 

 homologous in the two species — a point that will be considered later. 



With respect to the phenomena of linkage and ''crossing-over^' in related 

 species. — Most of Dr. Metz's work has centered around this feature. 

 In general the results show a series of linkage and cross-over phenomena 

 similar to those in ampelophila^ leaving no doubt that the genetic pro- 

 cesses are fundamentally the same in the two species. One especially 

 significant point of resemblance is in the dissimilar genetic behavior of 

 the two sexes. In ampelophila, as is well known, crossing-over occurs 

 only in the female; linkage is absolute in the male. This is a genetic 



