138 



Shall. 



change places, each uniting with the homolog of the other, as illustrated 

 diagrammatically in fig. 6, the result would be exactly the same as 

 if two determiners of the same type had originated independently in 

 these two pairs of chromosomes, for now two chromosomes (3 and 4, 

 fig. 6), each bearing the gene C, could enter into a single germ-cell, 

 while into the sister germ-cell would pass two chromosomes in which 

 the same determiner is absent. This same chromosome might carry 

 other determiners in a coupled system with C, all acting together as a 

 single unit, but it matters not in such a case how complicated the par- 

 ticular chromosome is, - - oversow 

 large a portion of the organic it 

 operates, or in how many diff,< -nt 

 physiological and morphological .fea- 

 tures it produces characteristic v^-ible 



i XA b c i i Xa fie j ; Xa b c j x a be i or invisible effects, the duplicate de- 

 terminer would represent exactly the 

 same coupled system and play exactly 

 the same role, because, although located 

 in a new position this chromosome 

 would really be the same organ and 

 not a new and independently originated 

 duplicate of it. 



There is another readily conceiv- 

 able method of rearrangement of genes 

 by which a duplication of determiners 

 would be brought about. While it is 

 now known that chromosomes do not 

 invariably unite to form a continuous 

 spireme, the work of numerous cytol- 

 ogists has made it probable that this 

 is the usual procedure. At one stage 

 in cell-division the chromosomes are united into an apparently continuous 

 strand, and at another stage the strand segments preparatory to the 

 formation of the daughter chromosomes. If the determiner C happened 

 to be located in the extreme end of one chromosome (3,3, fig. 7), it 

 seems a very reasonable assumption that the break in the strand which 

 forms the new chromosomes might once occur on the opposite side of 

 this determiner, so that instead of lying in the chromosome in which 

 it had been originally located (3), it would become a part of the reverse 

 end of the adjacent chromosome (4). The same result might come about 



Fig. 6. Hypothetical duplication of a 

 determiner by the displacement of a 

 chromosome. X represents the unhy- 

 pothesized residual constitution of the 

 chromosome; A, B, C, are hypothetical 

 Mendelian genes; a, b, c, simply call 

 attention to the absence of the genes rep- 

 resented by the corresponding capitals. 



