20 Mutations and Evolution. 



one chromosome will be extruded into the polar body, and if in the 

 spermatocyte the chances are small that more than one sperm 

 from the same spermatocyte will function. Still, as far as it goes, 

 the 12 single mutant individuals suggest that perhaps a single 

 sperm and hence a single chromosome of a pair has the new factor. 

 In the case of (Enothera lata and semilata, which arise through an 

 irregular heterotype division, evidence is available, for these 

 mutants sometimes occur in pairs, suggesting that the change 

 occurred in a pollen mother cell and that the two resulting 

 8-chromosome pollen grains both functioned. 



Mutations in Drosophila. 



The evidence for mutations in animals was rather scanty until 

 Morgan took up the study of the fruit fly, Drosophila melanogaster, 

 about 1910. The rapid breeding and easy handling of these flies 

 in large numbers, makes it an ideal form for genetic experiments, 

 and combined with this is the advantage that it has only four 

 pairs of chromosomes. It is not, therefore, surprising that in the 

 last decade, Morgan and his pupils have accumulated a mass of 

 breeding data, closely analyzed and correlated, which is unequalled 

 in any other organism. With plants growing only one seed- 

 generation a year, it would probably require 150 years to produce 

 an equal number of generations. 



The Drosophila work has therefore given us a look into the 

 constitution of the germ plasm such as no annual-breeding plant 

 or animal could furnish in a lifetime. The Mendelian behaviour, 

 sex-linked inheritance, and other features are similar to those 

 found in many other organisms, so there can be no doubt of the 

 wide applicability of the conceptions of mutations and the germ 

 plasm derived from these experiments. In many respects they are 

 in accord with those derived from the (Enothera work. Although 

 the simple Mendelian mutations in the latter are comparatively 

 few, we have already seen that the behaviour of the others can 

 probably be accounted for on the basis of sterility or lethal factors, 

 explanations which now enter into many features of the CEnotheras. 

 Again, the duplication of a chromosome in CE. lata and other forms 

 is paralleled by the phenomena of non-disjunction in Drosophila, 

 although there are certain differences which will be pointed out. 

 But in Drosophila the rapidity in breeding large numbers in many 

 generations has made it possible to carry the analysis of the germ 

 plasm much further than in any plant. 



