92 GENETIC VARIATIONS 



some of the chromosomes have become altered, so that they 

 no longer have the same effects as the corresponding genes in 

 the other chromosomes of the set. The actual occurrence of 

 such changes has not been observed, and their nature and 

 causes are unknown. Possibly it is in such polyploids that the 

 nature of evolutionary changes in the genes is most open for 

 investigation. 



In the direct attempts to observe genetic variations as they 

 occur in living organisms, one direction that has long been 

 followed lies in the relation of such variation to environmen- 

 tal action, and to the reactions of organisms to environmental 

 conditions. In the lowest organisms, those made up of single 

 cells, the occurrence of genetic variations has been observed 

 on a large scale, and of these many seem more readily classi- 

 fied under the topic of environmental action and reaction 

 than under that of gene mutation. An account of these will 

 be given in the remaining chapters. 



