PHYSIOLOGICAL 415 



Chromolipoids or Lipochromes. — This fourth group of pig- 

 ments is widely represented both among plants and among animals. 

 They are called "coloured fat-Hke bodies" or "fatty pigments", 

 but they show no great resemblance to fats beyond their solubility 

 in ether. The two yellow pigments, carotin and xanthophyll, which 

 accompany chlorophyll, are common chromolipoids, and they also 

 occur in many animals. Carotin gives the yellowish colour to butter 

 and xanthophyll occurs in the yolk of the bird's egg. The chromo- 

 lipoids are mostly reddish and yellowish pigments, and they occur 

 in many brightly coloured birds and fishes, where they are often 

 accompanied by melanins. The colouring matter of the yellow fat 

 in many animals, such as some lizards, has a chromolipoid nature. 

 Another good example is the reddish zoonerythrin ("animal red"), 

 which is common in many of the higher Crustaceans, such as prawns 

 and the Rock Lobster (Palinurus), which Victor Hugo called "the 

 cardinal of the sea". It is a widespread pigment, occurring, for 

 instance, in the red wattle above the eye of the grouse, and it is 

 chemically next door to carotin of carrots. The blue colour of the 

 common lobster is due to a compound of zoonerythrin with a 

 protein. When the protein is destroyed by heating, the free pigment 

 is left to give its familiar red colour to the boiled lobster. "And 

 like a lobster boiled, the morn From black to red began to turn!" 



There are many other animal pigments that cannot be included 

 in any of the groups referred to, such as the uric-acid pigments 

 of some butterflies' wings, the purple secretion of the dog-whelk 

 Purpura and of some other Gasteropods (the animal counterpart of 

 indigo), the red pigment of the cochineal insect (a distant counter- 

 part of the alizarin of madder), but it is perhaps more important 

 at present to emphasise the great classes we have mentioned — 

 the chlorophylls, the blood-pigments, the melanins, and the chromo- 

 Hpoids. In other connections we shall refer to some of the others, 

 such as the anthocyan of many flowers and of the withering leaves 

 — the flowering of the forest. 



Physiological Status of Pigments. — It would be a great gain 

 in biological interpretation if we could be more definite in regard 

 to the physiological status of organic pigments. Are they waste- 

 products, or by-products, or reserve-products? Is there any way 

 in which they may naturally arise in the course of the everyday 

 chemical routine or metabolism? In animals, are they sometimes 

 directly connected with the vegetable food? For if we can reach 

 some security in regard to the primary physiological significance 

 of a pigment, then we can proceed with more confidence to inquire 

 into {a) its internal utility in the life of the body, and {b) its 

 external survival- value in the struggle for existence. 



It is contrary to true scientific method to hurry towards generali- 

 sations before sufficient data have accumulated, yet tentative 



