RESPIRATORY PIGMENTS 159 



animals we see, following the gradual insistence of this 

 respiratory demand, the gradual evolution of a more 

 and more efficient oxygen-carrier. In the shellfish 

 the blood has only a quarter of that oxygen-holding 

 capacity which we possess. In the worms which live 

 where little air is available, the blood is richer, and 

 either green or red. In the vertebrates the blood is 

 always red, richer in oxygen-holding power, and meets 

 the demands created by the growth, bulk, and activity 

 of the body for a continuous supply of oxygen to all 

 the tissues. Of these demands some are more pressing 

 than others, and we can assign the deeper colour of 

 the more active muscles to the great stores of blood- 

 pigment heaped up for their use. Hence the darker 

 legs of a hen contrast with its breast, or the dark flesh 

 of game with the white flesh of poultry, And further, 

 if the generally accepted opinion is correct, that the 

 yellow, brown, and black pigments of hair and skin, 

 to which all the higher animals owe their colouring, 

 are kinds of effete blood-pigment removed from 

 the circulation and dumped down out of the way, 

 then we may look upon the blood as furnishing the 

 pigments which are worked up upon the canvas of 

 skin into such superb embroideries. 



We can thus broadly trace the rise of the red colour 

 of the blood to its source in some distant connection 

 between plants and animals, of which its chemical 

 affinity with plant-green is confirmatory evidence. 



Evolution of red and yellow fatty pigments. — A no 

 Jess complex and yet fascinating quest than this of 



