8 SALT-WATER FISHES 



important " secondary sexual character." The gemmeous 

 dragonet, for instance, and dusky skulpin, now well known 

 to be simply the male and female respectively of one and the 

 same species (^Callionymus), were long described as totally 

 different fish. This sexual colour difference is associated by 

 the majority of writers with the principle of natural selection, 

 the brightest-coloured males being for generations more 

 attractive to the females than those of less gay appearance. 

 Mr. Holt has, however, made another suggestion by way of 

 explaining the phenomenon, having found by experiment in 

 the aquarium that the yellow pigment in the female dragonet 

 is distasteful to pollack, gobies, and other kinds likely to 

 prowl on the breeding-grounds.* He therefore regards the 

 yellow female as particularly safe while engaged in the im- 

 portant work of reproducing the species, though, as the male 

 is frequently found in quantities in the stomach of cod-fish 

 and other larger forms, and as there are said to be three 

 times as many females as males, it is not easy to understand 

 in what way the particular protection of one sex only can 

 secure the continuance of the species. At the same time, 

 as will presently be shown, the milt of a single male is 

 capable of fertilising the eggs deposited by several females, 

 so that, individually, the male is perhaps somewhat less 

 precious. 



Spots, while characteristic of the salmon, trout, plaice, and 

 two of our dog-fishes, are not a very common character in 

 adult forms, but frequently occur in the young,*!* particularly 

 during the very early larval and post-larval stages. In some 

 species, even where absent in the mature, or full-grown, fish, 

 the spots persist to a later stage, as in codlings {Gadus). Bars, 

 on the other hand, which are retained throughout life by the 



* See Proc. Zool. Soc, 1898, p. 307. 



f Curiously enough, spots are also characteristic of many young 

 mammals. Some young lion cubs at the Zoo recently exhibited a 

 uniform scheme of spots on the body and limbs. 



