Introductory Sketch. 19 



to the light. It is in these alone that we find the tints and patterns 

 that render birds so strikingly beautiful, the underlying feathers 

 being invariably of a sober grey. Still further, many of the contour 

 feathers overlap, and the parts so overlapped, being removed from 

 the light are grey also, although the exposed part may be resplen- 

 dent with the most vivid metallic hues. A similar illustration can 

 be found in most butterflies and moths. The upper wing slightly 

 overlaps the lower along the lower margin, and although the 

 entire surface of the upper wing is covered with coloured scales, 

 and the underwing apparently so as well, it will be found that the 

 thin unexposed margin is of an uniform grey, and quite devoid of 

 any pattern. 



The law of structure, on the other hand, is an entirely new idea, 

 and demands more detailed explanation. Speaking in the broadest 

 sense, and confining ourselves to the animal kingdom, animals fall 

 naturally into two great sections, or sub-kingdoms, marked by the 

 possession or absence of an internal bony skeleton. Those which 

 possess this structure are known as Vertebrata, or backboned animals, 

 because the vertebral-column or backbone is always present. The 

 other section is called the Invertebrata, or backboneless animals. 



Now, if we take the Vertebrata, we shall find that the system of 

 colouration, however modified, exhibits an unmistakably strong 

 tendency to assume a vertebral or axial character. Common 

 observation confirms this ; and the dark stripes down the backs of 

 horses, asses, cattle, goats, etc., are familiar illustrations. The only 

 great exception to this law is in the case of birds, but here, again, 

 the exception is more apparent than real, as will be abundantly 

 shown in the sequel. This axial stripe is seen equally well in 

 fishes and reptiles. 



For our present purpose we may again divide the vertebrates 

 into limbed and limbless. Wherever we find limbless animals, such 

 as snakes, the dorsal stripe is prominent, and has a strong tendency 

 to break up into vertebra-like markings. In the limbed animals, on 

 the other hand, we find the limbs strongly marked by pattern, and 

 thus, in the higher forms the system of colouration becomes axial 

 and appendicular. 



As a striking test of the universality of this law we may take 

 the cephalopoda, as illustrated in the cuttle-fishes. These creatures 

 are generally considered to stand at the head of the Mollusca, and 

 are placed, in systems of classification, nearest to the Vertebrata; 



E 



