62 EVOLUTION, GENETICS, AND EUGENICS 



of the same breed. It would be a freak of nature were we to discover 

 any marked exception to the laws of heredity. Furthermore, our 

 ordinary daily contacts with other members of our own species have 

 taught us that, as a rule, the more closely alike people are, the more 

 closely are they related. We recognize that children of the same 

 family are more alike in their personal characteristics than are members 

 of the same race not so closely related. Whenever we see two people 

 whose resemblance is very great we assume a relatively close kinship. 

 Thus, everyone has had the experience of meeting two people so 

 strikingly alike that it is almost impossible to distinguish them apart, 

 and of immediately assuming that such persons are identical or dupli- 

 cate twins. Now the interesting thing about such twins is that they 

 are vastly more closely related than are ordinary brothers and sisters, 

 or even than are fraternal twins, who are only brothers and sisters 

 that happen to have been conceived and born simultaneously as the 

 result of the fertilization of two egg cells. For duplicate twins are the 

 products of the early division into two equivalent parts of a single 

 embryo derived from one fertilized egg. No closer kinship can well be 

 imagined than this, for the two individuals bear the same relationship 

 to each other as do the bilateral halves of one individual. 



The writer has had an exceptional opportunity of determining the 

 exact degree of resemblance existing between separate offspring de- 

 rived from a single egg. It so happens that a peculiar species of 

 mammal, the nine-banded armadUlo of Texas, always gives birth to 

 four yoxmg at a time. These quadruplets are invariably all of the 

 same sex in a litter and are nearly identical even in their finest ana- 

 tomical details, such as the numbers and arrangements of the plates 

 and scales in the armor and the numbers of hairs in a given area of 

 the skin. A detailed study of the embryonic history of this species has 

 proved beyond any question that in every case the four young in a 

 litter result from a very early division of a single embryo derived from 

 a single fertilized egg (see Fig. 77). Large numbers of sets of quadru- 

 plets were studied statistically to determine the exact degree of their 

 resemblance to one another. A comparison of over two hundred sets 

 revealed the somewhat startling fact that on the average they were 

 over 93 per cent identical (more technically, they showed a coefficient 

 of correlation of over .93). The remarkable closeness of this degree 

 of resemblance may be fully appreciated when it is realized that the 

 only structural resemblance belonging to this order of closeness is that 

 existing between the right and left antimeric halves of a single indi- 



