160 ANIMAL LIFE 



the history of blood-pigment is that of the yellow 

 and red colouring matters of the lower animals. We 

 all know the red colour of boiled shrimps, lobsters, 

 and crabs, and the change from blue to red which 

 these Crustacea suffer in the process is but an instance 

 of the breaking down by heat of a delicately poised 

 and unstable substance, and of its return to the basal 

 red pigment which we see in the change of phosphorus 

 from red to yellow on heating. 



But we are not familiar with the fact that such 

 red and yellow forms of a definite chemical substance 

 run like a thread through animal and plant organisms, 

 giving evidence of a similarity of constitution that 

 appears more and more strongly to the mind as we 

 grow acquainted with these unexpected relationships. 

 The yellow yolk of an egg, the red shell of a lobster 

 and yellow substance of a carrot contain a colouring 

 matter of the same chemical nature. The same 

 material colours the wax of our ears and the visual 

 purple of our eyes. The eyes of birds and reptiles 

 possess yellow globules, scattered amongst the part 

 most sensitive to light ; the eye of man and of ape 

 has its yellow spot or centre of acutest sight ; and in 

 the eyes of nearly all vertebrates there is a reddish 

 pigment which belongs to the same class of coloured 

 substances. In the skin of fish and that of frogs, 

 which arc so highly nervous as to be hardly less 

 susceptible to changes in their neighbourhood than the 

 eye itself, the same yellow and red pigment is abun- 

 dantly found, and there assumes the form of minute 



