136 THE STORY OF FISH LIFE. 



not we have to rely mainly on conjecture in 

 endeavouring to find an explanation of their 

 meaning. In some cases it would indeed seem 

 as if the now almost discarded recapitulation 

 theory received some support. That many of 

 the phases of these transformations have a 

 direct relation with the past there can be no 

 doubt ; on the other hand, many are as certainly 

 special adaptations belonging to, and necessary to, 

 the particular phase in which they appear. An 

 exceedingly instructive series of stages in develop- 

 ment is shown in the life history of the sword- 

 fishes. The young of Histiophorus, of the Pacific 

 and Indian Oceans, has been beautifully illustrated 

 by Dr Giinther, and these figures, by his kind 

 permission, have been reproduced here (fig. 13). 

 In the first stage, a fish of 9 mill, long, it is to 

 be noted that the jaws are of equal length and 

 both bear teeth ; above the eye is a series of short 

 bristles ; from the back of the head project, above 

 and below, long pointed spines. The dorsal fin 

 is long and low, the pectoral fin large and trun- 

 cated, whilst the ventral fins are represented 

 only by tiny buds. In the next stage, a fish of 14 

 mill, long, the dorsal fin has increased enor- 

 mously in size, whilst the spines projecting from 

 the back of the head are relatively shorter ; the 

 bristles above the eye have vanished ; the upper 

 jaw has grown slightly longer than the lower ; 

 the ventral fins, represented previously by buds, 

 have now increased to long slender filaments ; the 

 pectoral fin has changed its shape, and the pre- 

 operculum or gill-cover has increased greatly in 

 size. In the third stage, a fish of 14 mill, long, the 



