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, 



FIG. i. 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 



