534 TRANSACTIONS OF SECTION I. 



most efficient cause determining pigment production is the action of light. 

 Remembering that we are probably dealing with nuclear matter in general from 

 which this particular material has been split off and set aside by subsequent 

 causes, we can admit this postulate. The pigment-forming propensity of blood 

 is thus taken as probably due to the initial action of light upon nuclear material 

 placed near the surface of the body, and there exposed to the action of light. 



Our next step is to discover any probable fact which, favoured by natural 

 selection, might drive into the interior of the body nuclear matter that had been 

 so modified by light that it persisted in the formation of pigment; in other 

 words, Why should any pigment-forming reaction ever be removed from the 

 direct influence of light, and a valuable transformer of radiant energy be thus 

 driven into a position of disadvantage within the interior of the body 1 



Now let us consider the value of those particular instances of pigment 

 formation which have been allowed by natural selection to persist upon the 

 surface of the body. These successes represent experiments that have not been 

 detrimental to the general mass of chemical reactions which form collectively 

 what we call the organism, and we are entitled to ask, In what way are these 

 successes likely to differ from the failures? If we take the possibility that some 

 pigments convert all the light which they absorb into heat, and receive per unit 

 of surface a share of solar radiation measured as seven thousand horse-power per 

 acre, we have a picture that the body surface might thus be exposed at any one 

 time to the transformation of an excessive amount of energy. The square metre 

 of surface which might in the human body be exposed at one time to the sun 

 would, provided with such pigment, absorb in one hour as much heat as is pro- 

 duced by the whole body in twelve hours, and the temperature of the body 

 might be raised a further 20° C. by this means in one hour. It must then be an 

 important matter in which the risks of life maintenance have certainly acted 

 along the lines of natural selection, that such pigments transforming the total 

 energy they receive into heat must be driven from any place they have tem- 

 porarily occupied on the surface of living matter. As in the plant, in successful 

 cases this energy must be largely diverted into chemical work. It would not then 

 be surprising that certain modes of pigment formation have been eliminated, 

 and that certain other modes finding a utility of some other kind have been 

 retained by natural selection in seclusion within the interior of the body. Let us 

 take it that blood-pigment represents such a mode of reaction, and that its 

 influence is mainly to convert light into heat, and secondarily in some degree 

 to determine the separation of oxygen from certain compounds, thus also per- 

 forming some chemical work when under the influence of light. 



Now since it is also part of the general line of argument that it was ineffi- 

 cient in this chemical aspect, and on that account driven from the surface of the 

 body, it must be held as incapable of separating oxygen from more stable com- 

 pounds ; and we find an explanation for the fact that it is engaged upon unstable 

 compounds of oxygon, not absorbing much energy in the process of reduction nor 

 liberating much on oxidation. 



Since in regard to all such chemically dynamic pigments, with a utility 

 dependent upon their constant association with some molecular group in which a 

 corresponding reduction process can be effected, it will never be surprising to 

 find this group actually forming a constituent part of the molecule. It is, then, 

 not surprising to find these two qualities, pigment and unstable oxygen com- 

 pound, present in haemoglobin; nor to find in this special case that the secondary 

 process has assumed the position of major importance, and that haemoglobin 

 is no longer of use as a pigment so much as an unstable compound of oxygen. 

 Following this line of reasoning, there is nothing extraordinary in the discovery 

 that such pigments utilised as ' oxygen carriers ' within the interior of the 

 body are found in other situations than in blood — for instance, in the nerve- 

 cells of certain animals and commonly in skeletal muscle. Blood tissue represents 

 a special set of nuclear reactions possessed of this persistent quality in marked 

 degree. 



If it seems strange that the initial formation of blood in the embryo and 

 its maintained formation in the adult persist in the absence of light, let us 

 return to the instance of the eyeball. Of that instrument it was said that 

 although it was originally formed by light, yet in the mammalian embryo its 

 formation was continued in the absence of light. Here it was necessary to 



