532 EVOLUTION, GENETICS, AND EUGENICS 



No one can deny the equality of heredity and environment in one 

 sense: for both are absolutely essential. No organism can develop 

 without a basis of germinal substance; neither can any development 

 take place in the absence of the proper environment. The hereditary 

 material is, however, what we are and the environment and training 

 are what we have and what we do. What we are is really more funda- 

 mental in determining character. This is the opinion of the great body 

 of biologists. 



Geneticists have ceased to consider a man as a whole unit. He is a 

 highly elaborate complex of characters. It is futile to attempt to de- 

 termine whether a man as a whole is more the product of nature than 

 of nurture, but it is quite reasonable to attempt to measure the relative 

 contributions of nature or nurture to any single human character, such 

 as eye color, stature, brain power, well-defined idiosyncrasies. 



A natural form of experiment to test the relative potency of two 

 co-operating factors would be to keep one factor uniform and modify 

 the other, and vice versa. We might conceivably rear several indi- 

 viduals with identical heredity under varying environmental condi- 

 tions and compare the end results. Or we might place individuals of 

 unlike heredity under identical environmental conditions and compare 

 the end results. If the former tended to remain alike despite the en- 

 vironmental differences, it would appear that environment was im- 

 potent materially to affect heredity. Similarly, if individuals with 

 different heredity fail to grow more alike under the same environment, 

 a similar conclusion would be justified. The crux of such an experi- 

 ment is to discover individuals with identical heredity, and it is to the 

 insight and ingenuity of Sir Francis Galton that we owe the first crucial 

 test of the problem along the lines proposed. 



TWO KINDS OF TWINS 



Galton was perhaps the first to recognize that there are two kinds 

 of twins: identical twins and ordinary or fraternal twins. Ordinary 

 twins are merely brothers and sisters that happen to be born together. 

 Each comes from a separate fertilized egg, and they differ in their 

 hereditary complex as widely as do brothers and sisters in general. 

 Such twins may be both male, both female, or a male and a female. 

 Identical twins have an origin quite different from this. It is practi- 

 cally certain, though not as yet fully demonstrated for human beings, 

 that the two individuals are derived from the two halves of a single 

 egg which had been fertilized by a single sperm. They constitute, 



