FISHES 213 



through the water, we can scarcely attribute the colour 

 change in the male to special influences of light. Mr. Holt 

 mentions the suggestion which has been made in many 

 cases, that the pigment and guanin are due to an adven- 

 titious excretory process connected with genital activity. 

 But this suggestion is difficult to reconcile with the fact 

 that a similar excessive excretion does not occur in so many 

 other male fishes whose genital activity is, at least, equally 

 great. We are thus driven to the hypothesis, however 

 improbable it may seem, that the production of pigment and 

 guanin in the male is due to his sexual excitement, which 

 appears to be the only condition in which he differs greatly 

 from the female, and from other male fishes which do not 

 change colour in a similar way. By sexual excitement I 

 mean here the nervous excitement which is a recognised 

 physiological state, and of which the male dragonet affords 

 conspicuous evidence at the time of his courtship. Physio- 

 logical processes are known to be governed largely by 

 nervous impulses, and not merely the circulation, but the 

 excretory activity of the skin, are known to be influenced by 

 nervous action. Pigment and guanin are produced in the 

 skin by the secretory or excretory activity of the living cells. 

 That the marked excitement of the male dragonet's brain 

 should cause increased nervous stimulation of the skin is not 

 an extravagant supposition, and it is definitely supported by 

 the fact that the blue bands, if not the yellow, are flashed 

 out in more intense brilliance as the fins are raised in his 

 amatory rushes. When the question is regarded physio- 

 logically, instead of merely from the selectionist's point of 

 view, the significance of such reasoning as I have used 

 cannot be ignored. 



Such an explanation covers not only the development of 

 the coloration, but its partial diminution after the period of 

 sexual excitement is passed. Mr. Holt found that the yellow 



