WHITE— STUDIES OF INHERITANCE IN PISUM. 491 



both races, our systematists and comparative morphologists would 

 say, was the same character (in the absence of the experiments 

 cited above), but many geneticists would look upon them as two 

 genetically distinct characters, one of which is altered by a change 

 in environment. Acceptance of the conception of a character as 

 advanced above may mean a very radical change in the weight which 

 has been placed in the past on comparative morphology and taxo- 

 nomy as methods for studying the evolutionary history of plants 

 and animals. 



The Categories of Variation. 



Adopting the conception of a character as given above and as- 

 suming that plants and animals are made up of hereditary units or 

 factors, variations or character changes in organisms may occur in 

 three ways : 



1. Variations due to changes of environment. 



2. Variations due to "gain" or "loss" of factors or character de- 



terminers through crossing. 



3. Variations due to mutation. 



1. Variations due to changes in environment are perhaps most 

 clearly illustrated by the change from white flowers to red flowers 

 in Primula sinensis rubra following the change in temperature. So 

 far as experimental work goes, change of the same material from 

 one environment to another may take place repeatedly and each 

 time the materials react to the new condition in the same way. 

 Pink-flowered hydrangeas have blue flowers when placed in a soil 

 containing sufficient alum salts. The unbranched varieties of peas 

 are said to branch profusely under the climatic conditions of the 

 Pacific coast region of the United States. Cabbage refuses to head 

 in the tropics. Lock (54) found that seeds of certain pea varieties 

 sown in Ceylon in January and constantly watered produced re- 

 markably stunted plants, which flowered at half the usual height 

 (of seeds sown in November in Ceylon) and bore almost no seed. 

 Examples showing the direct influence of a changed environment 

 could be given by the hundred, did space permit. 



2. Changes due to crossing will be illustrated at length in the 

 part devoted to the genetics of Pisiun. 



