BIOLOGICAL RELATIONS BETWEEN CAROTINOIDS 195 



stratcd, that there is a universal dependence on the diet for the lipo- 

 c'hroine carotinoids of animal tissues. One cannot, in fact, account in 

 any other way for the brilliant carotinoid colorations among certain 

 birds, fish and other lower vertebrates. 



It must not be forgotten, however, that not all animal lipochromes 

 in the broad sense have been definitely identified as carotinoids. 

 There is at least one (possibly more) red lipochrome widely distributed 

 among animals, as shown in Chapter IV, which does not seem to have 

 an analogue among the plant carotinoids, although it seems to be 

 closely related to them. Such a pigment occurs in the feathers of 

 certain birds, in the salmon muscle, in the hypoderm of Crustacea and 

 elsewhere. Is it a plant carotinoid which has not yet been identified, 

 or is it a modified plant carotinoid? Either of these possibilities is 

 more rational than the possibility that it is actually synthesized by 

 the animals in which it occurs. The red pigment on the legs of the 

 pigeons is probably such a pigment. It is absent from the legs of the 

 young pigeons. In the color of its solutions this pigment strongly 

 resembles lycopin. The writer observed recently that the pigment 

 is strongly epiphasic between petroleum ether and 90 per cent methyl 

 alcohol, but its solutions show no clear absorption spectra. These 

 tests indicate a modified carotin, and yet there is little if any carotin 

 in the blood serum of the pigeon, the great bulk of the pigment being 

 xanthophyll as in the case of the fowl. The problem of modified caro- 

 tinoids in animal tissues is therefore an important phase of the general 

 hypothesis which must not be overlooked. 



There is still another phase of animal pigmentation from the caro- 

 tinoid standpoint which deserves consideration, namely, that caroti- 

 noid pigmentation is not universal, even among herbivorous animals. 

 The writer (1916) first called attention to some variations of this 

 kind among mammals, showing that practically no carotinoids occur 

 in sheep and goats and none in swine. Palmer and Kennedy (1921) 

 showed that there are none of these pigments in the albino rat. Ro- 

 dents in general, however, do not lack carotinoids, for one finds small 

 amounts in the guinea pig, as shown by van den Bergh, Muller and 

 Broekmeyer (1920). The rabbit is practically, if not entirely devoid 

 of carotinoids, although entirely herbivorous. The same seems to be 

 true of the dog, which is carnivorous, at least by preference. Cats, 

 however, contain traces, as shown by van den Bergh and associates 

 (1920). The general observation that some animals lack lipochromes 

 is not new, for Miss Newbigin used it as an argument against the 



