EVOLUTION, HEREDITY, EUGENICS 539 



have been able to locate factors in particular chromosomes, map- 

 ping out their relative positions.'^ Bridges has recently emphasized 

 the probability that the genes are possessed of tremendous catalytic 

 activity. 



Lethal Factors. — It has been found that in the fruit fly (Dro- 

 sophila) a number of lethal factors are in the X-chromosome. 

 Females with the lethal factor in both X-chromosomes will die; 

 and males with the lethal factor in their single X-chromosome 

 likewise die. The males that survive are those that lack the X- 

 chromosome and its lethal mutant factor. Lethal factors exist in 

 some other chromosomes besides the accessory or X-chromosomes. 



Non- Disjunction. — In 1913, Bridges discovered that unexpected 

 individuals obtained in his breeding experiments with white-eyed 

 Drosophila might be explained by assuming that the X-chromo- 

 somes did not disjoin and go to the two poles during maturation 

 division of the egg but remained attached to each other, and passed 

 to one pole. This would leave the other pole without any X-chromo- 

 some and in the resultant eggs one-half would possess two X- 

 chromosomes, while the other half lacked the X-chromosome. 

 Cytological evidence supported the hypothesis. 



Parthenogenesis. — In parthenogenesis the offspring may be 

 male, female, or parthenogenetic females. In the Hymenopera 

 fertilized eggs develop into females and unfertilized eggs into males. 

 In the Rotifers and Daphnids the fertilized eggs produce females, 

 but during parthenogenesis males come from small thin-shelled eggs 

 while females develop from larger glycogen-laden eggs. Even in 

 the frog, Parmenter has found that parthenogenetic eggs develop 

 into males and females. 



Evidence For and Against the Inheritance of Acquired Characters. 

 /. Are Somatic Acquirements Transferred as Such to the Germ 

 Cells? — Weismann cut off the tails of mice for nineteen generations 

 without effect on future generations. Blood transfusion refuted the 

 Pangenesis theory. There may be passive transmission of certain 

 dyes, syphilis, white diarrhea, pebrine, and Texas fever. There is 



17 But H. Przibram (Quanta in biology, Proc. Roy. See. of Edinburgh, vol. 49, pt. 

 3, pp. 224-231, 1929) suggests that crossing-over may be regarded as a "crosswise sub- 

 stitution of atoms or groups of atoms, the chain of genes remaining unbroken." (Con- 

 sult T. H. Morgan, The Theory of the Gene. Yale Univ. Press, New Haven, Conn., 

 1926, and C. E. McClung, The chiasmatype theory of Janssens. Qu. Rev. of Biol., vol. 

 2, PP- 344-366, 1927-) 



