SEXUAL CHANGE OF PARR 163 



sexes, or conditions of such autumn migrations, 

 although it may be surmised that in the first autumn 

 they are probably female fish. 



THE AIM OF NATURE 



There is, so far as I know, no evidence to disprove 

 this suggestion, while there are many arguments in 

 favour of its consideration. Nature and the laws of 

 natural selection are opposed to inbreeding, the aim 

 of Nature appearing always to favour the union of 

 the sexes of different stocks. The migration, there- 

 fore, of the male fish in the first spring will, on his 

 return from the sea, throw him into contact with 

 the female of another generation, and most prob- 

 ably of other parentage, and so on. It is* the belief 

 of many that each shoal of grilse ascending a river 

 is composed of the one or the other sex. 



As is only natural, a considerable diversity of 

 opinion exists, some authorities considering that the 

 female parr changes into a smolt before the male, but 

 this reverses the order of the physical development 

 of the sexes. 



It may be assumed that a minority of parr in any 

 one season's hatch migrate after one year's river life, 

 that a large majority of those left behind as parr 

 migrate after two years, and the small remainder, 

 probably the weaker ones, after the third year. 



But, whatever may be the approximate facts, food 

 and temperature will be a serious factor in the 



112 



