18 Colouration in Animals and Plants. 



to change at the shoulders and thighs, and also at each limb-joint. 

 In fact, if the cat be attentively remarked, it will clearly be seen 

 that the colouration or pattern is regional, and dependent upon the 

 structure of the cat. 



Now a cat is a vertebrate or backboned animal, possessing four 

 limbs, and if we had to describe its parts roughly, we should specify 

 the head, trunk, limbs and tail. Each of these regions has its own 

 pattern or decoration. The head is marked by a central line, on 

 each side of which are other irregular lines, or more frequently 

 convoluted or twisted spots. The trunk has its central axial back- 

 bone stripe and its lateral rib-lines. The tail is ringed ; the limbs 

 have each particular stripes and patches. Moreover, the limb- 

 marks are largest at the shoulder and hip-girdles, and decrease 

 downwards, being smallest, or even wanting, on the feet ; and the 

 changes take place at the joints. 



All this seems to have some general relation to the internal 

 structure of the animal. Such we believe to be the case ; and this 

 brings us to the second great law of colouration, namely, that it is 

 dependent upon the anatomy of the animal. We may enunciate 

 these two laws as folloAvs : 



I. THE LAW OF EXPOSURE. Colouration is primarily depen- 



dent upon the direct action of light, being always most 

 intense upon that surface upon which the light falls most 

 directly. 



II. THE LAW OF STRUCTURE. Colouration, especially where 



diversified, follows the chief lines of structure, and 

 changes at points, such as the joints, where function 

 changes. 



It is the enunciation and illustration of these two laws that 

 form the subject of the present treatise. 



In the sequel we shall treat, in more or less detail, of each point 

 as it arises ; but in order to render the argument clearer, this chapter 

 is devoted to a general sketch of my views. 



Of the first great law but little need be said here, as it is almost 

 self-evident, and has never been disputed. It is true not only of 

 the upper and under-sides of animals, but also of the covered and 

 uncovered parts or organs. 



For example, birds possess four kinds of feathers, of which one 

 only, the contour feathers, occur upon the surface and are exposed 



