NOTE ON SEXUAL DIMORPHISM. 41 



cause the rainbow-like show of colours. These ridges and grooves are an 

 exaggerated growth, the result of the stimulus conveyed to the vegetative cell 

 from the male generative cells. The excessive brilliance of colour of male 

 birds is essentially due to their sexual katabolic tendencies, that feature of all 

 male animals which results in the dissipation of energy. Fortunately, other 

 hws come into play which prevent the waste of energy ; in this instance, the 

 energy is conserved in the formation of those wonderful microscopic ridges and 

 grooves which give brilliancy to the feathers of male birds. I will not attempt 

 to explain why the energy should be conserved in this particular manner, but 

 I feel that it has been done for some useful purpose which will best be 

 understood by those who have made a special study of birds ; perhaps, 

 however, it is only another example of that beauty in design which we see in all 

 nature. 



In conclusion, may I hazard an explanation of the unusual frequency of ■white 

 feathers in male birds? You are aware that in a frog there are pigment cells in 

 the skin which are capable of contracting and relaxing. At rest they are 

 relaxed and the frog assumes a dark colour. During stimulation they are 

 contracted and the frog assumes a light colour. Now this power over pigment 

 cells in the skin is exemplified to a greater or less extent in all animals. May 

 it not be possible that the katabolic stimulus of the male generative cells, in the 

 case of the birds, has led to a contraction of the pigment cells. This contrac- 

 tion maintained for long periods would end, through the operation of the law 

 of disuse, in first the atrophy and later the complete disappearance of pigment, 

 a white colour would thus be produced. 



Coming now to consider Mr. Young's remarks on Mr. Dewar's paper I cannot 

 agree with him that Mr. Dewar has given a wider significance to the term 

 sexual dimorphism than is generally understood. Mr. Young's definition is very 

 inexact and eminently non-scientific. In the term sexual dimorphism must be 

 comprised all those differences which constantly distinguish the males from the 

 females in any one species. The subject, Mr. Young rightly remarks, is a 

 difficult one to discuss in a single paper chiefly because of its extensive appli- 

 cation to the whole of the animal and vegetable kingdom, even I maintain to 

 the lowest forms of life. Such a widely spread difference must essentially rest 

 on a great fundamental law operating in all cases. In this respect my opinion 

 differs entirely from Mr. Young. Moreover, I believe, it may be advantageous 

 to study this law in the single specialised class of birds, especially if the study 

 is made from an intimate knowledge, a specialist's knowledge, of the class ; a 

 knowledge not obtained from museums and books alone but from a study of the 

 birds in Nature. Such a special study prevents the tendency to arrive at gene- 

 ralisations which, if founded on incomplete knowledge of details, are sure to 

 mislead. 



I fail to understand Mr. Young's remark that " granted that abnormal growths, 

 colours, etc., take place during the breeding season they are more likely than not 

 to show themselves at those centres of muscular and nervous energy which are 

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