32 HEREDITY AND EUGENICS 



curves in pure and hybrid races, that in PoHsh rabbits 

 " the initial weight is less, the growth-rate lesS; 

 and the completion of growth comes earlier," all 

 these features combining to produce a smaller rabbit. 

 It is evident from these and other contradictory 

 results that further studies of size inheritance in 

 relation to rate of growth, etc., are necessar}- before 

 any final conclusion can be reached; but it is highly 

 probable that the same laws of size inheritance apply 

 to man as to mammals, whatever those laws may be.* 

 Castle's data provide strong evidence that in the 

 strains of rabbits he studied general growth factors 

 preponderated over any factors affecting only the size 

 of certain parts. Nevertheless, the effects of a genetic 

 factor are frequently confined almost entirely to one 

 organ, and we see no reason why this should not apply 

 to size factors as well as others. Wright (191 8) in 

 a statistical analysis of earlier measurements by 

 Castle, of a stock of rabbits w^hich gave strikingly 

 high correlations between skull and leg measure- 

 ments, brings out correlations which '' suggest the 

 existence of growth factors w^hich affect the size of 

 the skull independently of the body, others which 

 affect similarly the length of homologous long bones 

 apart from all else, and others which affect similarly 

 bones of the same limb." The five measurements 

 considered were length and breadth of skull, length 

 of humerus, femur, and tibia. Analysis of the rela- 

 tions shows that in a population of rabbits most 

 of the differences between individuals are those 

 which involve the size of the body as a whole. But 

 there is a certain amount of variation of each bone 

 length independently of all others measured. There 

 are also groups of bones, which vary together inde- 



* There is much evidence in man (see p. 212) that the activities 

 of various ductless glands, such as the thyroid and pituitary, 

 control the size and development. 



