N'o. 643] 



ORTHOGENESIS 



103 



Now it may be that originally hemoglobin possessed 

 only one of these peculiarities, so that its sole original 

 function was to carry oxygen. And aceordhigly one of 

 the most interesting questions of comparative physio- 

 logical chemistry concerns the respiratory function of the 

 blood. It would be a very important discovery to find a 

 kind of hemoglobin in which there is no specialized action 

 upon the transport of carbonic acid and upon the alkalin- 

 ity of the blood. But even if the earliest hemoglobin 

 were of such a nature, the first production of hemoglobin 

 would still seem to have been relatively an extremely dis- 

 continuous variation involving an unmistakable physio- 

 logical unit of great importance. 



It is true, and should be noted in order to avoid con- 

 fusion, that there has been a later evolution of the proc- 

 ess of oxygen transport. This has been commented on 

 by Barcroft as a result of his own importmit rcscaivlics. 

 It appears that variation in the electrolx t( s of the red 

 cells is accompanied by remarkable variation in tlie af- 

 finity of their hemoglobin for oxygen, and that this is the 

 explanation of the diflferences in the so-called oxygen dis- 

 sociation curves of the bloods of different species of mam- 

 mals. Here, as Barcroft points out, there is no difficulty 

 in imagining a process of adaptation, for the fact of 

 chemical discontinuity is not involved. It is a question 

 of changing proportions of the different substances. 



But in spite of the possibility of such phenomena, it 

 seems probable that there are, even in the human species 

 alone, a considerable number of important individual sub- 

 stances whose appearance in the eourst- of oru'anic evolu- 

 tion it is very difficult imagine. ox("e])t as a radical in- 

 novation. 



Accordingly, it mu-t hv apparent that in the present 

 state of our knowled.iiv, any theory which postulates con- 

 tinuitv in evolution is very n]isatisfa<-tory to th-e elieniist. 



Moreover, in this case one >eenis to l)e eonfi'onted with 

 an appearnace of discontinuity \vhieh does n(»t (h-])end. 



tude of a difference. 



