ON THE EVOLUTION OF HEMOGLOBIN. 85 



chemical differences of different species and genera of animals 

 and plants are certainly as significant for the history of their 

 origin as the differences of form. If we could clearly grasp 

 the difference of the molecular constitution and activities of 

 different kinds of organisms, Ave should be able to form a 

 clearer and better grounded judgment on the question how 

 they have been developed one from the other than we now 

 can from morphological considerations." As will be seen, 

 this vicAv of the question agrees remarkably with my own, 

 though he was led to it by more general considerations, 

 before such a striking illustration of the principle had been 

 discovered as almost to compel us to admit that there must 

 have been a gradual evolution of such important and com- 

 plex chemical compounds as haemoglobin, a substance which 

 contains the same constituent radical in the various stages 

 of its development. 



