44 The Life Story of the Fish 



"sexual-selection" theory. This theory applies especially to 

 those animals in which the male only displays the bright 

 colors. Influenced, perhaps, by the inability of women to 

 resist soldiers, a worldly fact so well known that even a 

 scientist could not escape it, Darwin pronounced the theory 

 that the female animal, throughout the ages, has been at- 

 tracted by bright colors. The most brilliant males, according 

 to him, have always had the best chance of getting mates and 

 offspring and of passing on their brilliance, and the result is 

 that it eventually spreads throughout the species. Experi- 

 ments have shown that in some animals at least Darwin's 

 psychology was wrong, and that the bright male coloring is 

 used not to seduce the female, but to frighten off rival males. 

 It is a warning rather than a promise, but the evolutionary 

 result is the same: the most brilliant male has the best chance 

 of getting a mate. 



In those species in which both sexes display the bright 

 colors, and especially in the jewel-fish, Darwin may have 

 been right. Recent experiments show that, given her choice 

 between a gentleman in bright courting red and one in the 

 drab neutral gray, the lady chooses the flashy clothes. The 

 bright coloring, in either sex, we may suppose, is a signal. It 

 attracts the attention. It announces to members of the other 

 sex that the wearer is ready and looking for trouble. 



A third kind of color-change sometimes occurs as part of 

 the normal growth of fish. In many species there is very little 

 color difference between the infant and the parent, but in a 

 few this difference is marked. In Bermuda, the male of the 

 *^lue-head," a common reef- wrasse about six inches long, 

 goes through so many color-changes during growth that for 

 years it was classified as several different species. A less strik- 

 ing, but more widespread and better-known, example is the 

 "parr-mark" of the salmon family. Rainbows, cutthroats, 

 browns, Atlantic salmon, Pacific salmon, brook trout, all have, 



