230 Experimental Transformation of Species 



different directions. There is clear evidence also that such 

 departures from the parental type may perpetuate them- 

 selves genetically. 



It is the study of the fundamental facts of heredity 

 since the beginning of the Twentieth Century that has 

 yielded the most convincing evidence of the occurrence of 

 evolution. At the same time it has furnished the most prom- 

 ising basis for understanding how the changes come about. 

 It is these studies that answer for us the question, How can 

 a thing be the same and yet not the same? 



The Problems of Heredity 



The problem of heredity is present in essence to every 

 mature person who has kept his eyes open and who, without 

 coming to any satisfactory conclusion, has asked the funda- 

 mental questions: Why are children more like their parents 

 than they are like other people? Why are not the children 

 of the same parents more alike than they are? The fact of 

 continuity of kind is quite as familiar as the fact of varia- 

 tion — and so far as common knowledge is concerned, quite 

 as great a mystery. It is only our familiarity with the end 

 result, namely that each bears according to its kind, that has 

 made us stop inquiring as to how the result is brought about. 

 Only confusion can come from stressing either heredity or 

 variation at the expense of the other. Both are essentially 

 different aspects of the same fact. 



Every mature person has also considered more or less 

 persistently the questions regarding the source of variation: 

 Does A differ from B because of different ancestry, or do the 

 two differ because of different environments or upbringing? 

 Again we recognize the fact of heredity and the fact of 

 environment. Many people would like to have categorical 

 answers to the question, Which of these two facts is more 

 important or more potent? Nothing is gained, however, 

 by stressing either at the expense of the other. Living 

 things being what they are, heredity and environment are 

 equally inescapable conditions of life. Neither concept has 



