HUMAN INHERITANCE 199 



of body, or of neck, and of different combinations of 

 these may be present. Height is a growth phenome- 

 non depending amongst other things on the time at 

 which the growth of the bones, especially the long 

 bones of the leg, stops, and this in turn is, to some 

 extent at least, connected with the time of sexual 

 maturity, which depends again on the time of func- 

 tioning or the amount of secretion ^^roduced by some 

 of the glands of internal secretion (testes or ovary, 

 thyroid, pituitary, etc.). These inter-relations have 

 made the study of growth very difficult especially 

 when the determination of the rate of growth or its 

 cessation is connected with internal organs that are 

 seldom seen or measured. Nevertheless there is 

 nothing in these complications that precludes the 

 possibility that the ultimate source of the variability 

 is due to genetic factors. 



There is little accurate information at present as 

 to the number of factor differences that are involved 

 in the inheritance of hair color in man. The changes 

 in color that take place during the life of the indi- 

 vidual renders its study difficult. An individual child 

 may start with flaxen hair, later have brown hair, 

 and in old age become white-headed. Three genera- 

 tions living at the same time may show these differ- 

 ences. A complete record would then have to extend 

 over several years; for, hearsay evidence as to the 

 color of the hair of the grandparent when in middle 

 age may be inaccurate and the futiu'e color of the 



