SOCIAL AND WORLD ASPECTS 213 



glands as well are concerned in the determination of 

 skin pigmentation, and we have already indicated 

 some of the other racial differences which they 

 probably control as well. If, then, in the various 

 races hereditary differences in the activities of these 

 glands of internal secretion are concerned, it may be 

 expected that the results of racial crosses will be 

 very different from what they would be if there were 

 direct determiners for the various individual visible 

 characters. For each endocrine (ductless) gland 

 has various activities and affects the development oi 

 many parts of the body. In crosses between races, 

 whole complexes of characters w^ould then be ex- 

 pected to be inherited or modified together. If the 

 difference between a white and a black skin is due to 

 the greater activity of the suprarenals in the white 

 race, then it would appear that quantitative differences 

 in the activities of these organs are the units really 

 involved in white-black crosses. The laws of quan- 

 titative inheritance are by no means clearl}^ under- 

 stood at the present time, nor are even the facts of 

 colour inheritance in mankind entirely agreed upon 

 (see p. 52) ; but if they are due to inherited differences 

 in endocrine activitv, we have here the basis for a 

 further analysis and understanding of the combined 

 phenomena of blending and segregation in the colour 

 inheritance of racial crosses. 



The thyroid is considered by Keith to be anthro- 

 pologically the most important of all the endocrine 

 glands. The thyroid, like all these glands, is believed 

 to throw off two types of substances,* one which is 



* It may be questioned whether there are really two types of 

 substances given off, one early in development and the other 

 after maturity. It appears rather that a germinal change in, for 

 example, thyroid production causes morphogenetic changes 

 which will be inherited, while a functional derangement of the 

 same kind occurring later will produce different results and not 

 be inherited. 



