THE ADDITION OF DETAILS 87 



same hereditary factors, and if one can find identical 

 twins which have been separated in early life and 

 brought up in different families, one can measure the 

 relative importance of heredity and upbringing by 

 finding how much they do or do not resemble each 

 other. So far only a fairly small number of pairs of 

 separated identical twins have been carefully studied, 

 so that the conclusions are not very certain, but, on 

 the whole, the information which has been collected 

 emphasizes the importance of heredity, even for 

 shaping intellectual and emotional qualities. 



Some animals have specialized in producing many 

 embryos from one egg. One species of Armadillo, for 

 instance, always has four identical quadruplets. 

 Other animals, like some of the parasitic wasps 

 which lay eggs in caterpillars of other insects, pro- 

 duce enormous numbers of young from one egg. In 

 such cases the egg falls to pieces at an early stage in 

 its development, and each piece then goes on to make 

 a whole embryo. We may perhaps find out how it is 

 done, and then we could control twin production at 

 will, and produce a large number of babies from an 

 egg with a particularly good set of hereditary factors. 



Different Ways of Building Up Similar Structures 



When an embryo reaches the mosaic stage the real 

 elaboration of detail begins. The ways in which the 

 different organs are built up are too manifold and 

 too complicated to be summarized in such a short 

 book as this. There is only room for a description 

 of how the main outlines of the animal are laid 



