53^ EVOLUTION, GENETICS, AND EUGENICS 



vessels. There are on record no data as to the degrees of resemblances 

 and differences between twins in which the placental connections are 

 known. Hence there are but few definite data upon which to measure 

 the relative proportion of hereditary and environmental factors in the 

 case of human twins. 



One interesting set of comparisons has, however, been made by 

 Frederick Schatz, who more than any other writer has gone into the 

 details of the consequences of human twinning. This author had a 

 large amount of gynecological material at his disposal and was able to 

 deal statistically with the resemblances and differences between one- 

 egg and two-egg twins before and after birth. He found, strangely 

 enough, that one-egg twins, though they have the same hereditary 

 composition, are considerably less alike in size and weight at about the 

 middle of pregnancy that are two-egg twins. The cause of the great 

 inequality is associated with the fact that the placental blood vessels 

 of the two individuals struggle for supremacy in the single placental 

 area and undergo more or less extensive anastomoses, so that one twin 

 sends blood over to the other and vice versa. There is great oppor- 

 tunity for unfairness of give and take and, as a rule, one twin is dis- 

 advantaged at an early time. At the time of birth, however, the differ- 

 ences in size and weight of the one-egg twins are essentially equal to 

 those of the two-egg twins; which means that the one-egg twins have 

 grown more alike and the two-egg twins have grown less alike in spite 

 of all the environmental factors that have been at work to make the 

 one-egg twins less alike and the two-egg twins more aUke. This is in- 

 deed a test of the relative potency of heredity and environment, and 

 the result is greatly in favor of heredity; but we have information only 

 about matters of size and weight, which are perhaps the least valuable 

 characters for comparison because they are so notoriously changeable 

 and seem to depend so largely upon nutrition, a factor of the environ- 

 ment. The fact that even these characters seem to be so definitely 

 governed by heredity is at least not in favor of the environment side 

 of the discussion. 



ARMADILLO QUADRUPLETS 



On the whole, then, we may say that human twins furnish little 

 or no conclusive evidence of the exact relative values of the factors of 

 heredity and environment. What we need to find is some species, 

 preferably a mammal, in which we know that one-egg twins occur and 

 in which there is no doubt that we are dealing with individuals with 

 identical heredity. Some years ago I was fortunate in finding just 



