37 



NOTE ON SEXUAL DIMORPHISM. 

 By L. C. H. Young, B.A. 



In criticising Mr. Dewar's paper, I would like to say in the first place that 

 he gives a much wider significance to the term than is generally accepted. The 

 difference in the necks of a mare and a horse would not ordinarily be described 

 by the term, any more than the average difference in breadth of shoulders in 

 man and woman. By sexual dimorphism is generally understood the presence 

 in one sex of some abnormal character which, if man did not know to the 

 contrary, might lead him to suppose they were different species. The point 

 may be easily illustrated by the two best known species of Felis. In the tiger 

 there is no sexual dimorphism, although the sexes are abundantly distinct in 

 average measurements ; the mane of the male lion is. however, a clear instance 

 of the phenomena under discussion. 



Sexual dimorphism is one of the largest and most difficult questions in the 

 whole of Biology, and it is as impossible to deal with it properly in one paper 

 as in ten minutes criticism. Moreover, it is found in all but the lowest orders 

 of creation, and although I would not suggest that the same laws necessarily 

 govern it in all, it only tends to confuse us to try and deal with it in one 

 class to the exclusion of others. 



An enormous amount of literature has been written on the subject, and in so 

 far as Mr. Dewar is criticising Darwin's theory of sexual selection, he is 

 whipping a dead horse ; for no one, I think, accepts this one of Darwin's theories 

 at present except in a limited number of cases as a working hypothesis for want 

 of a better. But we must not make the mistake of regarding Wallace's theory 

 as contradictory to it. One regards the question from a physical and the other 

 from a metaphysical point of view. 



Granted that abnormal growths, colours, etc., take place during the breeding 

 seasons they are more likely than not to show themselves at these centres of 

 muscular and nervous energy which are nearest the seats of excessive vitality, 

 i.e., the head, lungs, and caudal regions. But this does not answer the question 

 why. 



Of the much that has been written in answer to this question " why " only two 

 theories are important — one is Darwin's that they are acquired because they 

 are beautiful, while the other side maintain that they are of the nature of a 

 hereditary diseased growth or hypertrophy resulting in almost every case from 

 some habit, generally that of fighting, characteristic of the males during the 

 breeding season. This of course begs the whole question whether characters 

 acquired during lifetime can be transmitted to descendants, but there is no 

 time to discuss that now, or even the theory itself, in any detail. £uff.'ce it to 

 say that, if the hypotheses are granted, it is quite wonderful how the theory fits 

 in with the known facts, and especially in cases where the dimorphism takes 

 a peculiar direction it has generally been found that the creature has some 

 peculiar method of fighting, etc., affecting the hypertrophied region. Stags' 

 horns of course give a conspicuous example of the theory, while cock's combs 



