like is not only modified by the influence of external cir- 

 cumstances, but to a still greater extent by that tendency 

 to variation which is manifested in all animals, and which 

 Darwin regarded as the greatest argument in favour of his 

 theory of evolution. There are in fact no two beings pre- 

 cisely alike ; brothers and sisters present many differences ; 

 and even were they formed according to the undisturbed 

 force of hereditary transmission, endless diversifications 

 would result from inevitable diversities of proportion of 

 transmission, parental, as well as ancestral. The operation 

 of hereditary tendencies is perpetually disturbed by innu- 

 merable circumstances too subtle for us to appreciate, and 

 yet capable of producing varieties infinite alike in extent 

 and degree. How these spontaneous differences occur we 

 cannot tell ; but that individual varieties are transmissible, 

 and capable of producing new races in domesticated animals 

 by artificial selection is indisputable ; and in the human 

 family the tendency to spontaneous variation i even still 

 greater. 



I now propose to view heredity in its physiological, psy- 

 chological, and pathological aspects, and in the space at 

 my disposal to give at least some general idea of this most 

 important biological law. 



There are few natural phenomena more interesting and 

 marvellous than those affecting inherited peculiarities of 

 feature, figure, character, constitution, habits, etc., and their 

 perpetuation, and the numerous facts of hereditary trans- 

 mission are all the more wonderful when we contrast them 

 with other and better-known phenomena, which experience 

 and scientific progress have enabled us to group under cer- 

 tain definite laws. The laws of planetary motion are recog- 

 nised, and to a great extent understood ; the systematic 



