GENETIC STUDIES OF RABBITS AND RATS. 19 



GENERAL OR LOCAL SIZE FACTORS. 



In the light of these facts, what should be our attitude toward the 

 view (expressed by some students of human heredity) that mixed 

 races are likely to contain individuals with physical maladjustments 

 and disharmonies? On this view it is assumed that there are inde- 

 pendent genetic determiners affecting the size of the different parts 

 of the body. If so, when races of different size are crossed, recom- 

 bination of genetic factors will in later generations produce some 

 parts larger, others smaller, than the general average of the races 

 crossed. This will result in disharmonic combinations, as, for 

 example, hearts too large or alimentary tracts too short for the bodies 

 in which they are found. Is there any real ground for such appre- 

 hension? I think not. There is in health a perfect correlation in 

 size of each part with every other part of the same body and with 

 the size of the body as a whole. The modern sciences of embryology 

 and physiology tell us why this is so. It is (1) because development 

 of the individual from the fertilized egg is so largely epigenetic, each 

 stage growing out of its immediate predecessor; and (2) because it is 

 so largely controlled by internal secretions. 



The view of the genetic independence in size of the various parts 

 of the body is a sporadic relapse into preformationism, such as was 

 perhaps excusable to the Grecian mind when, without the control of 

 observational or experimental science, it fancied animals to arise by 

 chance coming together of arms, legs, and other parts which originally 

 floated free and unconnected in primordial slime. The day for such 

 preposterous ideas is past. There may be valid reasons why mixing 

 of the more distinct human races should not be advocated, reasons 

 perhaps sociological, but there need be no fear that an animal or- 

 ganism will result whose parts are not properly coordinated. We 

 are only beginning to understand the mechanism of such coordination, 

 through studies of the ductless glands, but it is already clear that 

 such coordination and control are very complete and are adequate 

 for the production of harmonic organisms in the widest racial crossing 

 possible in the animal kingdom. 



I think that a strong probability has been established that the 

 genetic factors which affect size in mammals are general in their action, 

 exclusively so. In this last particular I dissent from the view ex- 

 pressed by Wright (1918) who, from a statistical examination of 

 MacDowell's data, concluded that the genetic factors indicated 

 were in minor part special and local in action. Davenport has advo- 

 cated a similar view concerning the inheritance of human stature, 

 but on grounds which appear to me to be inadequate for two reasons: 

 first, because of the imperfect character of his data, and secondly 

 because of unwarranted deductions from them. Measurements of 

 the several elements of human stature made on the living subject 



