THE PHILOSOPHY OF XESURUS 289 



geonfish sometimes with one, sometimes with the 

 other. There seems, however, Httle doubt that the 

 butterfly-fish and the surgeons are much closer. A 

 matter of some fifty miUion years ago, in the 

 Eocene, there swam a family of fishes — the 

 Pygaeidae — in which these two groups were 

 brought very close together indeed (Fig. 43) . 



We need here concern ourselves only with the 

 character of the mode of defense, which is curi- 

 ously different in the three living groups. In 

 general, that of Xesurus sets it rather apart from 

 the others, whose dependence is upon the anterior 

 spines of the dorsal fin. 



In the butterfly-fish these are very long and 

 strong, but not especially modified, and grade into 

 the posterior, lesser spines of the dorsal. Both in 

 the aquarium, where we kept black-fronted butter- 

 fly-fish alive for several weeks, and near the bottom 

 of the shallow shores where these exquisite fish 

 lived their lives in pairs, I watched them fence. 

 The simile which comes to mind is of a pair of full- 

 grown tahr on a steep Himalayan mountain side. 

 I have watched these splendid wild goats through 

 long-range glasses, each shifting into most graceful 

 poses as he feinted and made passes with his horns, 

 either in play or in grim earnest, kneeling, swing- 

 ing sideways, rearing lightly into the air with fore- 

 legs bent under and horns playing like rapiers. 



This morning when I was diving, a large sea 

 bass passed close to where a pair of butterfly-fish 

 was feeding, and as it approached, one of the two 

 went out to meet it. Every great spine gradually 



