NOTE ON SEXUAL DIMORPHISM. 39 



The latter part of Mr. Dewar's paper does not call for much comment as 

 regards the main theory under discussion. I would merely remark as regards the 

 assumption of a white garb by certain birds during the breeding season, that 

 white is not a pigment* but the absence of it and the concentration of the vital 

 energies in another direction is just as likely to cause an absence of pigment a 

 excessive vitality in other species might produce abnormal pigment. 



L. C. H. YOUNG. 



NOTE ON SEXUAL DIMORPHISM. 

 By Captain W.G. Liston, I.M.S., M.D., F.R.S.E. 



I listened with much pleasure to Mr. Dewar's paper on sexual dimorphism 

 and cannot refrain from making a few remarks thereon, especially in view 

 of Mr. Young's criticism of the paper. 



I feel constrained to believe that were Darwin still alive he would no longer 

 attribute sexual dimorphism to sexual selection. Already before his death 

 he saw that he had not paid enough attention to the part played by use 

 and disuse, environment, etc., in modifying the order of Nature. 



Mr. Dewar seems to me to be on the right track when he insists on the 

 importance of the organs of generation as a factor in the development of sexual 

 dimorphism. Indeed, I think, we must take a broader view than either Darwin 

 or Wallace did, and consider sexual dimorphism as a phenomenon based on the 

 fundamental idea of sex. 



When we contemplate the universe as a whole, we are made aware of 

 the action of two opposing principles. Newton established the law of 

 motion that to every action there is an equal and opposite reaction. The 

 naturalist well knows that plants break up the carbon dioxide gas given 

 out by animals, appropriating to themselves the carbon and letting free 

 the oxygen, which can again be used by the animals. Here the plants 

 break down what the animals have built up. Again, plants build up from 

 simple inorganic elements, complex protoplasmic molecules which furnish food 

 for herbivora, and after assimilation by them are given back by excretion 

 as simple inorganic bodies which in their turn serve as food for the plants. In 

 this instance animals break down bodies which the plants have built up. 

 In the chemical and physical world the great law of the conservation of 

 energy teaches us that while there may apparently be a breaking down there is 

 at the same time a building up. The familiar example of the burning 

 candle will call to mind what I mean. Indeed, wherever we look there 

 is a breaking down and building up process going on — there is katabolism 

 associated with anabolism. 



I believe that it was Thomson and Geddes who first pointed out that the 

 essential difference in the sexes depends on the fact that the male element has 

 always katabolic tendencies, while the female element has essential anabolic 



* Iu writing out my notes for the Journal I, of course, accept the Chairman's correction 

 that where I used the word " colour " here I meant " pigment. " 



