MAMMALS WITH CONTRASTED COLOURS 149 



carapace was elaborated, became confirmed by ages of usefulness. 

 In other words, the nerve-centres acquired the habit of acting 

 that way. 



Then when, from whatever cause, the calcareous matter of the 

 exoskeleton disappeared, the nerve-centres continued that "sartie 

 action, which resulted in pigment pictures of the ancestral plate- 

 rosettes on the supple and elastic unarmoured skin. This is not 

 all, for the margin of the carapace was also pictured by contrast of 

 colour between the upper (ancestrally . armoured) and the lower 

 (unarmoured) surfaces ; the margin remaining pictured even when 

 all traces of rosetting had disappeared. 



Then innumerable further modifications in the atomic constitu- 

 tion of the nerve-centres, in which natural selection no doubt has 

 played a great part, have resulted in all the varied colorations of 

 mammals we see. 



Of course it is impossible to say what atomic changes in the 

 nerve-centres produce a dark back and a light abdomen in some, 

 or a light back and dark abdomen in others, any more than it is 

 possible to make out why one animal turns out albino, and another 

 melanoid, or why in one very dark grey Horse, with only few 

 vestiges of spots, its mane and tail w&iepure white. 



Some might perhaps fancy that they account for changes in 

 coloration by saying, ' Oh ! that is an albino,' or ' that is a case of 

 melanoid variation.' But this in no way explains its cause any 

 more than if one said it is a ' whitino,' or a ' blackino ' ! We are 

 at present wholly unable to say why, in a litter of black-and-tan 

 puppies, one comes out wholly tan, or tan and white ; or why, in a 

 litter of tan puppies, one comes out wholly black, or wholly white. 

 All we can say is that these phenomena do occur. 



This we may say ; in the Jaguar these changes of colour occur 

 in the components of the rosettes themselves. What I conjecture 



