90 ACQUIRED CHARACTERS sec. 



The physical cause of the dark colouring of the integument is 

 obviously that in consequence of the greater flow of blood to the 

 skin under the action of light and warmth, of their stimulation, 

 pigment is deposited there. Possibly in some cases greater 

 moisture has contributed to the result. An increased supply 

 of blood can effect the deposition of pigment in particular 

 parts of the skin without the influence of light. This happens, 

 as we know, in woman round the nipples, in man on the 

 scrotum, in both sexes in the arm-pits and round the anus. 

 Dr. Passavant, known for his travels in the Cameroons, states 

 in his doctorate dissertation that in the negroes just those 

 parts which are least exposed to light are darkest, e.g. the 

 arm-pits, and he argues from this fact against the conclusion 

 that licjht is the cause of darkness of colour in man. This 

 apparent contradiction finds a very simple explanation in the 

 above considerations. 



Like the pigmentation of the human skin, and pigmenta- 

 tion in general, the formation of leaf-green (chlorophyll), the 

 green colour of plants, depends also on the influence of light. 

 But here the colour is not inherited by the offspring, for when 

 these are reared in the dark they remain colourless. 



The profound action of light on the whole physical 

 constitution of the plant-body, on the whole physiology of 

 plants, is shown by the fact that many tropical plants, as for 

 example the South American species of Bougainvillea, in 

 European hothouses either do not bloom at all or only 

 incompletely in spite of all application of warmth, on account 

 of the deficiency of light.^ 



And who would deny that action when he only thinks of 

 the influence which light exercises on the direction of growth 

 of plants, and therewith on their whole form ? 



to northern races of mankind, wlio are not constantly exposed to this 

 action. 



'^ Cf. Charles Martin's edition of the PhilosopMe Zoologique of Lamarck. 

 German, Jena, 1876. (In the biographical introduction by Martin.) 



