20 DISTRIBUTION OF HEMOGLOBIN AND ALLIED SUBSTANCES 



That hemoglobin may have its source in chlorophyl has been shown 

 by MacMunn, who found in Helix pomatia substances intermediate between 

 chlorophyl and hemoglobin. That substances like hemoglobin derivatives 

 found as constant constituents of certain of the lower organisms do not 

 represent degradation products seems likely, inasmuch as it is hardly 

 conceivable that a molecule so exceedingly complex as hemoglobin should 

 have appeared suddenly. Unfortunately, our data are so insufficient that 

 we can not trace this probable synthesis or possible degradation step by 

 step, and for the same reason we can not show the causes of the extraor- 

 dinary gaps in the distribution of either hemoglobin or its derivative-like 

 bodies in the invertebrates. 



CHEMICAL NATURE OF TYPICAL RESPIRATORY SUBSTANCES, ETC. 



We will now consider the functional properties of protein and other 

 components, with especial reference to the probable misconception of the 

 specificity of the role of the iron in the respiratory phenomenon. 



Specific respiratory substances are doubtless essential constituents of 

 all living organisms, except probably only certain of the very lowest forms 

 of plant and animal life. They may be divided primarily into two groups, 

 metal-bearing and metal-free. The former may contain manganese, copper, 

 or iron, and they are normally, as far as known, colored; the latter are 

 colorless, as the achroglobulins of Griffiths. That undiscovered colorless 

 metal-bearing respiratory substances may exist in some of the lower organ- 

 isms, and even in the higher forms, seems more than probable. That such 

 bodies do exist as abnormal substances (for instance, in the form of de- 

 colorized hemoglobins or some close modification) has been ascertained 

 by a number of investigators. It has been recorded, for instance, that 

 hemoglobin may be crystallized and the crystals completely decolorized 

 without change of form or elementary composition, and that the decolorized 

 substance can even be recrystallized without alteration. Hemocyanin 

 (reduced) is colorless. The chlorocruorins bear, as has been shown, a striking 

 likeness to hemoglobin, spectroscopically and chemically, yet these sub- 

 stances may appear green, yellow, carmine, red, brown, etc. From the 

 foregoing it is obvious that it is not the mere presence of the metal, nor the 

 kind of metal, per se, that gives to the molecule its coloration, but the 

 peculiar arrangement of the atoms or groups of the molecule. That color- 

 less non-metal-bearing respiratory substances do exist has been shown by 

 Griffiths. Entirely apart from this, it must be admitted that the existence 

 of a large number of absolutely or practically colorless organisms, inverte- 

 brate and even vertebrate, in which the interchange of O and C0 2 in the 

 blood goes on quite actively, clearly indicates that colorless (metal-bearing 

 or non-metal-bearing) respiratory substances must have a wide distribution 

 in animal life. Moreover, it is suspected by the physiological botanist that 

 there may exist colorless plastids which are actively photosynthetic, like 

 the chlorophyllous plastid. 



The discovery by Griffiths of colorless metal-free respiratory sub- 

 stances in Patella, Chiton, Tunicata, and Doris (which have respiratory 



