:66 



NATURE 



[September 14, igu 



a P°- ' .advantage within the interior of the 



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 possi- 

 bility 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 produced 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, trans- 

 forming the total energy they receive into heat, must be 

 driven from any place they have temporarily 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 performing 

 some chemical work when under the influence of light. 



Now since it is also part of the general line of argu- 

 ment that it was inefficient 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 compounds ; and we find an explanation for the 

 fact that it is engaged upon unstable compounds of oxygen, 

 not absorbing much energy in the process of reduction nor 

 liberating much on oxidation. 



Since, in regard to all such chemically dynamic pig- 

 ments, with a utility dependent upon their constant associa- 

 tion 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 compound, 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 extra- 

 ordinary 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 commonlv in skeletal 

 muscle. Blood tissue represents a special set of nuclear re- 

 actions 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 think of some 

 replacement of one cause by another, and not difficult to 

 adopt such a suggestion, since even in the initial process 

 it was probable that light produced its effects subsequent 

 to transformation into some other form of energy, such as 

 electricity. In that particular case this idea of forces, and 

 substituted forces, in action, is capable of being formu- 

 lated in fashion readihj understood, because of "the ease 

 with which we can think of arrangements in gross parts 

 being determined by such forces. Here in this new case 

 we are, however, thinking of parts .«f a different order of 



no. 2185, vol. 8;] 



magnitude, a fact which I can best illustrate by reference 

 to a single red blood corpuscle occupying a one-tenth 

 millionth of a cubic millimetre, and containing in a one- 

 hundredth part of that space as many molecules of haemo- 

 globin as there are present red blood corpuscles in one 

 cubic millimetre of blood — that is to say, live millions. 



Now there is, in reality, no difficulty in considering 

 some electrical agency as limited in its action to the 

 minute dimensions in which each pigment-forming reaction 

 is in process ; some electrical machine such as, for example, 

 might be energised by electrons derived from the dissolved 

 molecules of a pigment-salt ; such a machine as might be 

 capable of transforming both light and heat into electrical 

 energy, and which would maintain a process in the absence 

 of light at the cost of energy obtained in the form of 

 heat. 



When thinking of the persistence of such reactions as 

 this, initiated in this way by the action of certain primary 

 causes that are then subsequently removed without any 

 cessation of the reaction, we are concerned with one of 

 the fundamental properties of living matter. Everywhere 

 in living matter numerous instances of this property are 

 being discovered, as in the study of immunity and of 

 protection from infection. Nor is there reason to believe 

 that this persistent quality of such variations will not 

 finally be explained in terms of physical chemistry. The 

 main characteristic of living matter is that it contains 

 machines formed by electrolytes distributed upon the com- 

 plex surfaces of matter in a state of colloidal solution and 

 in the presence of competitive* solvents, and that such 

 machines are multiplied within it. Some of thi se 

 mechanisms are arranged and perfected by the action of 

 physical conditions operative on the surface of living 

 matter, as, for instance, light. Some by energy derived 

 from internal sources, but in a form that embodies con- 

 ditions originally derived from the surface. Some are 

 primarily due to internal disturbances in the equilibrium 

 of these complex solutions produced by those chemical 

 reactions which take place there. 



Now, returning to the grosser characteristics of blood, 

 we find it possessed of other characters curiously reminis- 

 cent of the surface of the body, and especially of glandular 

 invaginations from the surface. Thus it is everywhere 

 confined by cells spread upon its surface, the endothelium, 

 which limit its relationship to the general mass of the 

 interior. Its new-formed cells are again passed into an 

 internal core covered by these surface cells, and from this 

 situation, except as a result of violence, they do not , 

 We might, in fact, compare the blood to a gland in which 

 the red blood corpuscles were seen as a secretion 00 

 ing a lumen which represents the original external su 

 of the body. I do not wish to lay any emphasis on this 

 point except in so far as it renders clearer this thought : 

 that blood covered by its endothelium represents a single 

 tissue which tends, like any gland, to grow into 

 interstice of the body, where the conditions of mechanical 

 pressure permit. I shall render the point clearer by sav- 

 ing that the blood capillaries are no more and no 

 than blood-tissue. 



In its early days this blood-tissue, or, if you will, this 

 capillary network, is pushed into each portion of the 

 by pressure due to its growth. In its later stage the 

 tissues surrounding it, which form the muscular coat of 

 the heart and the walls of the blood-vessels, are arranged 

 into an externa! mechanical system providing a new 

 pressure, which still further tends to pu-h the blood-tissue 

 into every available space, a process such as. for example, 

 takes place in tumour development and in the granulation 

 tissue present in wounds. 



It is a general postulate that cells long exposed 10 

 constant conditions may come to be stamped by those con- 

 ditions. Special change takes place from the time when 

 the blood grew onward by pressure of its own growth to 

 the time when this movement is more clearly detertnined 

 by the mechanism of the circulatory system, and divergent 

 results occur in different localities of the blood-tissues 

 which can be attributed to the differences in these causes 

 of onward motion. Thus where growth is the leading 

 cause of this progressive motion, as, for example, in the 

 development of bone, the blood-tissue is Inter found occupy- 



