1090 THE THEORY OF TRANSFORMATIONS [ch. 



snout. We need not stop to consider such extreme cases as the eel 

 or the globefish (Diodon), whose ways of propulsion and locomotion 

 are materially modified. But it is certainly curious that no sooner 

 do we try to correlate deformation in one direction with deformation 

 in another, than we are led towards a broad generalisation, touching 

 on hydrodynamical conditions and the limitations of form and 

 structure which are imposed thereby. 



Our simple, or simplified, illustrations carry us but a little way, 

 and only half prepare us for much harder things. But interesting 

 as the whole subject is we must meanwhile leave it alone ; recognising, 

 however, that if the difiiculties 'of description and representation 

 could be overcome, it is by means of such coordinates in space that 

 we should at last obtain an adequate and satisfying picture of the 

 processes of deformation and the directions of growth. 



A Note on Pattern 



We have had so much to do with the study of Form that pattern 

 has been wellnigh left out of the account, although it is part of 

 the same story. Like any other aspect of form, pattern is correlated 

 with growth, and even determined by it. A feather, for example, 

 which is equally and equidistantly striped to begin with, may have 

 this simple striping transformed into a more complex pattern by 

 the unequal hut. graded elongation of the feather. We need not go 

 farther than the zebra for a characteristic pattern of stripes, nor 

 need we seek a better illustration of how a common pattern may 

 vary in related species. 



A zebra's stripes may be broad or narrow, uniform or alternately 

 dark and pale — these are minor or secondary diversities; but the 

 pattern of the stripes shews more conspicuous differences than these, 

 though the differences remain of a simple kind. A zebra's stripes 

 fall into several series. One set covers the neck, including the mane, 

 and extends backwards over the body and forwards on to the face; 

 and these "body-stripes" are all that the extinct Quagga possessed. 

 On the head they are interrupted by the ears and eyes, and end at a 

 definite vertex on the forehead: from which, however, they run 

 down the face in pairs, of which the first pair of all may jr may not 

 coalesce into a single median stripe (Fig. 553). A second series 

 runs up the foreleg, and where it meets the body we have the 



