xviil THE COMPARISON OF RELATED FORMS 



749 



axes are inclined at an angle of 70° ; but this is now (as far as can 

 be seen on the scale of the drawing) a very good figure of an 

 allied fish, assigned to a different genus, under the name of 

 Sternojjtyx dicvphana. The deformation illustrated by this case 

 of Argyropelecus is precisely analogous to the simplest and 

 commonest kind of deformation to which fossils are subject (as 

 we have seen on p. 553) as the result of shearing-stresses in the 

 solid rock. 



Fig. 375 is an outline diagram of a typical Scaroid fish. Let us 

 deform its rectilinear co-ordinates into a system of (approximately) 

 coaxial circles, as in Fig. 376, and then filling into the new system, 

 space by space and point by point, our former diagram of Scarus, 

 we obtain a very good outline of an allied fish, belonging to a 



Fig. 375. Scams sp. 



Fig. 376. Pomacanthus. 



neighbouring family, of the genus Pomacanthus. This case is all 

 the more interesting, because upon the body of our Pomacanthus 

 there are striking colour bands, which correspond in direction 

 very closely to the lines of our new curved ordinates. In like 

 manner, the still more bizarre outlines of other fishes of the same 

 family of Chaetodonts will be found to correspond to very slight 

 modifications of similar co-ordinates; in other words, to small 

 variations in the values of the constants of the coaxial curves. 

 In Figs. 377 — 380 I have represented another series of Acantho- 

 pterygian fishes, not very distantly related to the foregoing. If 

 we start this series with the figure of Polyprion, in Fig. 377, we see 

 that the outlines of Pseudopriacanthus (Fig. 378) and oi-Sebastes or 

 Scorpaena (Fig. 379) are easily derived by substituting a system 

 of triangular, or radial, co-ordinates for the rectangular ones in 



