GENE MUTATIONS AND EVOLUTION 69 



tain curious relations in the occurrence of chromosome breaks 

 and injuries. 



While under radiation, the chromosomes are long threads 

 bearing the genes at intervals. In well studied organisms, the 

 order in which the different genes occur on the thread is well 

 known, so that maps of the genes have been constructed. 



When through the action of radiation there occurs in a 

 chromosome a break at a certain point, it is an extraordinary 

 fact that there is usually or always in the same cell another 

 chromosome break, at some distance from the first mentioned 

 one. This other break may occur at another point in the same 

 long chromosome, or in another chromosome of the cell. This 

 is another strange fact, worthy of full consideration. 



Chromosome breaks occur in very few of the cells, abso- 

 lutely considered. The fact that there usually occur two breaks 

 in one cell cannot be due to chance. Whatever causes one 

 break causes also another. The chance of a single electron's so 

 striking the chromosome as to cause two breaks some distance 

 apart is so minute that it can be excluded. 



And now another strange fact. After the two breaks have 

 occurred, the pieces separated by the breaks usually unite by 

 their broken ends, often in a different order from before. In 

 the case of a single chromosome that has broken in two places, 

 thus yielding three pieces, the three pieces often reunite in 

 such a way that the middle piece is reversed in position 

 (Figure 11, 5). That is, the order of its genes is the opposite 

 of that which prevailed before the break and reunion; this 

 can be determined by breeding tests. A great number of such 

 inversions of the middle part of the chromosome are known 

 — the order of the genes reversed in the middle piece, though 

 not in the two end pieces. This is an extraordinary phenom- 

 enon ; it cries out for explanation. 



