4 EVOLUTION AND DISEASE. 
speck. These collections of pigment are due to the 
irritation caused by the presence of a parasite. In 
tigers, lions, monkeys, and sheep, similar pigmented 
spots are occasionally found in the lungs around 
parasites. In man, horses (especially grey horses), 
and dogs, tumours of an inky-black colour, called in 
consequence melanotic, are occasionally met with. All 
these formations of pigment are purely pathological. 
Under normal conditions, however, cuttle-fish (Octopus, 
Fic. 1.—A Dace with spots of black pigment due to the irri- 
tation of a parasite (Mus. Royal College of Surgeons). 
and Sepia), possess an ink-bag from which, when these 
animals are irritated, an ink-like pigment, sepia, can be 
ejected in such abundance as to colour the surrounding 
water to the extent of a cubic yard or more, and under 
cover of this dark cloud the cuttles escape from their 
enemies. 
The close relation existing between physiological and 
pathological processes is shown in an interesting manner 
