NOT IDENTICAL TWINS MAY DIFFER IN RATE OF GROWTH AS WELL 



AS IN LOOKS 



While it would be a mistake to overlook the influence of proper food upon the develop- 

 ment of a child, it is just as great an error not to realize that children inherit different 

 reactions with regard to the same foods. If one child develops more slowly than 

 another, it may be because of its heredity and not because of any environmental 

 influence. (Fig. 22.) 



customary environment produce meas- 

 ureable modifications? In order to 

 prove that a modification is due to an 

 environmental change it is essential that 

 the heredity (or chromosome) factor 

 should be a constant. This requirement 

 may be sometimes attained by using a 

 large number of individuals whose 

 average heredity may be reasonably 



424 



supposed to be the same. For exam- 

 ple, there cannot be any doubt but that 

 the striking dififerences in the growth 

 of dogs, sheep, and other animals, when 

 given greater and smaller amounts of 

 glandular extracts — thyroid, etc. — are 

 really caused by these environmental 

 forces ; i. e., a difiference not caused by 

 chromosome differences. 



