47 8 



EVOLUTION, GENETICS, AND EUGENICS 



only about as similar as ordinary brothers or sisters. A typical pair 

 of fraternal twins is shown in Figure 90. 



The origin of identical twins. — While there is no doubt in the minds 

 of biologists that human identical twins are derived from the division 

 of a single egg or embryo, it has never been possible to demonstrate 

 this directly. Early stages in the development even of ordinary single 

 human embryos are exceedingly rare, and there are no specimens 



Fig. 91. — A pair of typical identical twins, showing no genetic differences. 



known that are young enough to show the beginnings of normal twin- 

 ning. Hence our belief that identical twins come from a single egg 

 is based on a mass of indirect, but none the less conclusive, evidence. 

 In the first place, it so happens that the entire developmental 

 history has been worked out for another mammal, the nine-banded 

 armadillo, an animal which bears one-egg identical quadruplets at 

 nearly every birth (see Fig. 52). The peculiarities of both egg and 

 uterus are very similar to those of man. The known steps of embry- 

 ology of the two species accord so closely that it is believed that the 



