332 CHROMOSOMES IN HEREDITY : PHYSIOLOGICAL 



lost the capacity for reaction, although it has retained the secondary 

 capacity for reproduction which is conditioned by the reactions of 

 the other genes associated with it in the complement of a higher 

 organism. Now the capacity for reaction cannot have been born 

 in the gene after the capacity for reproduction. It must rather 

 have been born first. We can therefore take the two major steps 

 in the evolution of the gene to be : — 



reaction -^ reaction + reproduction -^ reproduction. 



We also learn from inert genes that reaction and mutation are 

 bound up with one another, for inert genes have lost these two 

 properties together. Perhaps, therefore, natural mutation is an 

 abnormality of reaction. This assumption agrees with the observa- 

 tions that rates of mutation, like rates of reaction, are both geno- 

 typically and environmentally controlled. They are conditioned 

 by the substrate. 



Muller and Gershenson (1935) take a different view of the change 

 to inertness. But however the change takes place it is clearly 

 irreversible. W^e might therefore expect that inert genes would 

 be found very generally ; the widespread occurrence of super- 

 numerary fragments indicates that thi is so, and that inert genes 

 are usually concentrated near the centromere. In this position 

 they cannot so easily be got rid of, and this is a possible reason why 

 they are found here. We may say that they are clogging the genetic 

 system with useless but inevitable material. On the other hand, it 

 is possible that they are preserved near the centromere when 

 crossing-over is reduced or absent in this part, as they are in the Y 

 chromosome. If Heitz's method of identif\dng inert genes proves 

 satisfactory as an approximation it will help in solving this problem. 



5. THE GENE 

 We are now in a position to enquire more closely what we mean 

 by the gene. The differences between organisms are found to 

 behave in crossing-over as though they had a linear order. They 

 are known to be determined by changes in the chromosomes. 

 The chromosomes consist of particles, the chromomeres, shown 

 by observation to be arranged in linear order. Therefore it is 



