of the Salmon in Fresh Water. 163 



methylated spirit a small amount of a pigment which resembled the 

 pigment obtained from the liver of the salmon, the same pigment bein- 

 present in traces in a muscle extract. This suggests the possibility 

 that the salmon obtains the yellow pigment of its muscle from food in 

 association with fat, and that part of this pigment is modified to form 

 the red. In the lobster there is some reason to believe that the yellow* 

 pigment is capable of being transformed into the red, and the conditions 

 under which the two exist in the salmon suggest the possibility of a 

 similar transformation there. As to the possibility of transference of 

 yellow pigment from one organism to another, there is some evidence 

 apart from the case of the salmon. Thus Poulton (Proc. Roy. Soc., Lon- 

 don, liv., pp. 417-430; see also Nat. Sci., vol. viii., pp. 98-100) has shown 

 by experiment that certain caterpillars derive their pigments from their 

 food. Again, it is not uncommon to find the fat of sheep and cows dyed 

 .a deep yellow colour. According to some authorities, this occurs quite 

 sporadically without known cause, while according to others special 

 foods, notably maize, are the important agents. I have examined the 

 yellow pigment of maize and compared it with pigment from yellow 

 fat. The maize pigment gives the lipochrome reaction, faintly with 

 sulphuric acid, distinctly with nitric, while the fat pigment gives no 

 lipochrome reaction. In other respects, in tint, in solubility, and so on, the 

 pigments closely resemble each other-. This fact, taken in combination 

 with Mr Poulton's experiments, seems to me at least to prove the 

 possibility of the transference of these pigments from one organism to 

 another, and therefore to suggest such an origin for the yellow pig- 

 ment of the salmon. 



This suggestion, however, gives rise at once to the difficulty that 

 unless these three organisms can be shown to possess some common 

 physiological peculiarity, then we are forced to the conclusion that 

 all yellow pigments in animals are derived from their food a conclusion 

 for which there seems little evidence. Further, if the presence of pig- 

 ment in the food is the only condition necessary to produce pigmented fat, 

 it is difficult to understand why such coloured fat should not be 

 universal in herbivorous animals, for all green parts of plants contain 

 also a certain amount of yellow pigment. 



It seems to me, however, that it is possible to point to a peculiarity 

 possessed in common by the salmon, domesticated cattle, and cater- 

 pillars, namely, the habit in each case of ingesting food in excess of the nor- 

 mal requirements of the organism at the time of feeding. That this is so in 

 the case of sheep and cattle undergoing the artificial process of fattening is 

 obvious. Again, in both caterpillars and salmon the life-history is sharply 

 divided into nutritive and reproductive periods, the periods occurring, 

 respectively, once in the life of the caterpillar, and annually in the case 

 of the salmon. During the nutritive period in both cases there is a 

 large ingestion and deposition of fat, which later furnishes the energy 

 used up during the reproductive period. It seems to me not unreason- 

 able to suppose that while an organism which ingests a moderate amount 

 of coloured fat is able to utilise or eliminate the pigment, and so deposit 

 colourless fat in the tissues, one in which the ingestion of this coloured 

 fat is excessive may be unable to do this, and so store coloured fat. In 

 Poulton's caterpillars part of the pigment was eliminated with the faeces, 

 which suggests that elimination is the natural fate of the pigment. If 

 this explanation be correct, then it would follow that the reason for the 

 coloration of the fat in sheep fed on maize must be that the ordinary 

 diet contains as much pigment as it is possible for the organism to deal 

 with, and the further addition of the pigmented fat of maize causes 

 merely deposition of slightly altered pigment in the tissues. 



