CHAPTER XII 



CONCLUSION 



It is not easy to attempt a general assessment of significance of the 

 considerable amount of knowledge concerning the carotenoids which 

 is now available and which has been presented in the foregoing pages. 

 With regard to the occurrence, distribution and identification of caro- 

 teniods, the situation is very satisfactory but with regard to formation 

 and function, knowledge is rudimentary. 



Most of the pigments described have been unequivocally identified 

 although, inevitably, there are doubts concerning some. Perhaps the 

 most common defect met in the literature (and not only in carotenoid 

 studies) is the branding with fresh names of pigments, the uniqueness 

 of which is open to considerable doubt ; in other words, the possi- 

 bility that a pigment has previously been described has not been 

 eliminated completely. More than one name for the same thing can 

 not only cause considerable confusion but also sometimes lead to the 

 perpetuation of errors — for usage rather than appropriateness often 

 determines survival in scientific literature. 



It is now well established that carotenoids are manufactured de novo 

 only in the plant world. Animals take in these pigments as part of 

 their food and then deal with them in one or more of a variety of ways, 

 viz. : the pigments may be unselectively absorbed ; they may be 

 selectively absorbed ; they may be altered in some way, generally 

 by oxidation, before storage in special organs ; or they may, if their 

 structure permits, be converted into vitamin A. 



The tendency to dispose of carotenoids in a particular way depends 

 to a considerable extent on the position of the animal in the evolu- 

 tionary tree. Mammals, birds, some amphibia and probably fish, 

 possess the specialized ability to convert certain carotenoids into vita- 

 min A. Frogs and probably primates, including man, are unusual in 

 that they tend broadly to absorb carotenoids unselectively. Cows and 

 horses, are typical of a group of species which in the main selectively 

 absorb carotenes without altering them. Birds and fish, on the other 

 hand, tend to store unchanged xanthophylls rather than carotenes. In 

 both these groups, however there is a tendency to oxidize a small 



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