102 



THE AMERICAN NATURALIST 



[Vol. LVI 



a single chemical individual in order to illustrate a par- 

 ticular point. 



This substance is the sole means of transporting more 

 than a small amount of dissolved oxygen in the blood of 

 those species which contain it. It is, therefore, apparent 

 that it may be thought about from the evolutionary point 

 of view, much as one thinks about an organ. I believe 

 that the success of Aristotle's system of classification 

 justifies this view. But while it is easy to think of the 

 gradual evolution of an organ as something which can 

 not be regarded as appearing at any point in the evolu- 

 tionary process, being related by a process of continuous 

 differentiation to something which was certainly not the 

 same organ in an ancestral species, there is not the 

 slightest evidence for anything of the sort in the case of 

 hemoglobin, and it must seem to most chemists nothing 

 less than fantastic to assume such a continuous evolution 

 of a substance more and more closely approaching hemo- 

 globin. Moreover, it is almost as difficult to imagine such 

 a thing from the standpoint of a biologist, and it is cer- 

 tainly true that any given organism either does or does 

 not contain a substance which is capable of forming a 

 loose chemical com])ination with oxygen. 



But the difficulty in the case of hemoglobin is more 

 serious than this, for it has been found that hemoglobin, 

 like other organs, has more than one function. It has, in 

 fact, at least three ; for it is the sole means of transporting 

 oxygen, almost the sole means of liberating carbonic acid 

 in the lung and absorbing it in the tissues, and the instru- 

 ment of the final delicate adjustment of the alkalinity of 

 the blood. The last two functions depend upon the same 

 property in the hemoglobin molecule, but this property is 

 a (liffor(Mit one from that which enables hemoglobin to 

 coinbiiic with oxyucn. We are, therefore, here confronted 

 with \hi' t.i-lv nt' ima.uining the origin of a chemical sub- 

 stance, (juitc dilTcrcnt in its nature from any other known 

 substance, which possesses two chemical peculiarities, 

 and which, as a result of these two peculiarities, performs 

 three highly important functions. 



