BIOLOGICAL SIGNIFICANCE OF HEMOGLOBIN 191 



When I say that the molluscs preferred hsemocyanin to haemo- 

 globin, naturally I am using no more than a figure of speech. 

 It is, of course, not my meaning that in the evolution of the snail 

 any conscious selection of pigments was made, yet the figure of speech 

 stands for a very striking phenomenon — a phenomenon as striking as 

 if a deliberate choice had been made and the die cast in favour of 

 the copper compound. For in some snails you get the copper and 

 iron compounds side by side. The hsemocyanin is the respiratory 

 pigment found in the blood-vessels; the helicorubin, which is a 

 hsemochromogen, is thrust from the liver into the ahmentary canal. 

 Why it is not the other way round we do not know. Why is not the 

 copper pigment thrust out and the iron one retained and circulated 

 as haemoglobin in the blood-vessels? Here again we do not know. 

 Whatever may be the cause, it appears that there is no case in which 

 haemoglobin is foiuid in the presence of haemocyanin, though it is not 

 unknown for hsemocyanin to be found in the presence of the haemo- 

 chromogen. And it may be that the protein of hehcorubin is too far 

 removed from globin to make the formation of haemoglobin from 

 helicorubin possible ; at least the spectroscopic evidence rather lends 

 itself to that reading of the situation. We shall see in a moment that 

 there are a great many different haemoglobins, the difference being, 

 as far as we can tell, in the globins; yet all these globins are so nearly 

 alike that the spectra of the haemochromogens derived from them are 

 indistinguishable. Not merely are they of the same type but the 

 positions of the bands are the same within the Umits of the error 

 of reading. The spectrum of helicorubin is a haemochromogen spectrum , 

 inasmuch as it is of the haemochromogen type, but it could be 

 distinguished from globin-haemochromogen by a difference in position 

 of the bands. 



When we come to the haemochromogens we find that the field over 

 which we have to look is as broad as it was narrow in the case of 

 haemoglobin. Of these the one which claims our attention in the first 

 instance is cytochrome. Cytochrome has been so fully dealt with 

 at the commencement of this book that here it is only necessary to 

 recall the fact of its almost universal distribution over the animal 

 kingdom, of its apparent function as a catalyst, and of its being a 

 haemochromogen, if not a group of haemochromogens. 



When one tries to get behind the haemochromogens, to find out, as 

 it were, what is their ancestry, one is at present in the dim ages of 

 chemical evolution. One looks in several directions — Are there simpler 



