538 EVOLUTION, GENETICS, AND EUGENICS 



distributional factor associated with the mechanism of cell division and 

 body formation. This factor involves some profound inaccuracy in 

 the supposedly exact mechanism of mitosis which should equally 

 distribute hereditary materials to all cell products of a single zygote. 

 Whatever this third factor is, call it developmental inaccuracy if you 

 will, it tends to interfere with the degree of resemblance between twins 

 and is consequently to be thought of as explaining a considerable part 

 of the failure of the quadruplets to show complete identity. In spite 

 of this factor and in spite of whatever influence may be exerted by en- 

 vironment, the average potency of heredity is about 93 per cent, as 

 compared with 7 per cent for both environment and the third or distri- 

 butional factor. We have no basis for estimating how much of the 7 

 per cent is environmental and how much distributional, but I suspect 

 that very little of it is environmental. In the case of the armadillo, 

 therefore, we are forced to the conclusion that environmental factors 

 such as may be concerned in the development of bodily characters are 

 largely ineSective in modifying heredity. The environmental differ- 

 ences of position, variations in food, or any other developmental 

 variables are not suflliciently great to disturb at all seriously the equal- 

 ity of the quadruplets, all of which start out with a common heredity. 

 We know nothing about the mental qualities of the armadillo. Whether 

 his mentality is more or less plastic than his body is hidden from us. 



TWINS THAT ARE MODIFIED BY THE ENVIRONMENT 



Quite the opposite conclusion with reference to the influence of en- 

 vironment upon heredity is brought out by a beautiful experiment of 

 Nature upon cattle twins. Twins are very rare in cattle. When they 

 do occur they are two-egg twins with a different hereditary make-up 

 from the start. They may or may not be of the same sex. When they 

 are both of the same sex, they are merely like ordinary brothers and 

 sisters; but when they are of opposite sexes, a male and a female, the 

 male is always normal and the female is nearly always an anomalous 

 creature, partly male and partly female, called a freemartin. This 

 much has been known for some time, but it remained for Professor 

 F. R. Lillie to work out the details and to solve the problem. Using 

 the unexampled opportunities of the Chicago stockyards, he obtained 

 large numbers of cattle twins at all stages of their development. The 

 situation is this: Sex in cattle, as in other animals, is inherited. An 

 individual is zygotically determined as either a male or a female when 

 the egg is fertilized. The uterus of cattle is bicornate, consisting of two 

 long horns communicatine with a common region. One egg usuallv 



