Heredity and the Origin of Variations. 169 



differential growth of organs. When an organ is well 

 exercised, there is increased nutrition and increased growth 

 of tissue, muscular, nervous, glandular, or other. When 

 an organ is, so to speak, neglected, there is diminished 

 blood-supply, diminished growth, and diminished functional 

 power. The development of a complex activity would 

 necessitate a complex adjustment of size and efficiency of 

 parts, involving a nice balance of differential growth de- 

 pendent on delicately regulated nutrition. What is the 

 evidence that adjusted nutrition can be inherited ? 



With regard to man, there is some evidence which bears 

 upon this subject. Mr. Arbuthnot Lane, in his valuable 

 papers in the Journal of Anatomy and Physiology, has shown 

 that certain occupations, such as shoemaking, coal-heaving, 

 etc., produce recognizable effects upon the skeleton, the 

 muscular system, and other parts of the organization. And 

 he believes* that such effects are inherited, being very 

 much more marked in the third generation than they were 

 in the first. Sir William Turner informed Professor Herd- 

 man that, in his opinion, the peculiar habits of a tribe, such 

 :as tree-climbing among the Australians, or those natives 

 of the interior of New Guinea whose houses are built in 

 the upper branches of lofty trees, not only affect each 

 generation individually, but have an intensified action 

 through the influence of heredity, t 



Mr. Francis Galton's results mainly deal with human 

 faculty; and though faculty has undoubtedly an organic 

 basis, I do not propose to consider the evidence afforded 

 by instinct, intelligence, or intellectual faculties in this 

 chapter. Mention should, however, be made of the in- 

 teresting results of his study of twins. Twins are either 

 of the same sex, in which case they are remarkably alike, 

 or of different sexes, in which case they are apt to differ 

 even more widely than is usual with brothers and sisters. 

 The former are believed to be developed from one ovum 



* See Journal of Anatomy and Physiology, vol. xxii. p. 215. 

 f See Professor Herdman's Inaugural Address, Liverpool Biological 

 -Society, 1888. 



