FISHES 



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male may be the real cause of its modification. The elongation 

 or hypertrophy of the anterior rays of the dorsal fin may be 

 caused by the frequent agitation of these rays under the 

 influence of sexual excitement, and the heightened activity 

 of the nervous system in that condition may account for the 

 greater development of coloration in the skin. 



A sexual difference of colour is stated by Darwin, on the 

 authority of A. Agassiz, to occur commonly in the species of 

 the Chromidie, which are fresh -water fishes allied to the 

 Labrida? or Wrasses. It appears that the males are more 

 brightly coloured, whether they guard the eggs, as some do, 

 or not. Some of the males in this family, like the North 

 American Pomatis, build nests for the eggs and keep guard 

 over them, while some which occur in the Sea of Galilee are 

 said to carry the eggs in their mouths, like Arius among the 

 Siluridse. Probably, however, the guarding of the eggs has 

 nothing to do with the brighter colour of the males, which is 

 most likely, as in Callionymus, entirely due to the nervous 

 excitement of the male in his endeavours to effect fertilisation. 

 The Geophagus, mentioned by Darwin as a species in which 

 the male carries the ova in his mouth, and develops a pro- 

 tuberance on his forehead during the breeding season, is one 

 of the Chromidae, and in all probability, if the behaviour of 

 the fish were known, the two facts are connected together, 

 the protuberance being probably due to some friction caused 

 by the action of the male in getting the ova into his mouth. 



Another family allied to the Wrasses is that of the 

 Embiotocidse or surf- perches of the coast of California. 

 The females of these are all viviparous, but I have not yet 

 found any sexual differences described in them except that 

 of size, the females being much larger than the males. That 

 this is not the necessary consequence of the viviparous habit 

 is shown by the case of mammals, in which the males are 

 usually the larger. 



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