312 PHYSICAL PROPERTIES 



other words the haemoglobin crystals of any genus are isomorphous. 

 In some cases this isomorphism may be extended to include 

 several genera, but this is usually not the case unless, as in the 

 case of the dogs and foxes, for example, the genera are very 

 closely related. On the other hand the oxyhaemoglobin obtained 

 from the same species always crystallizes in the same form, al- 

 though often with different habit when obtained by different 

 methods of preparation. But upon comparing the haemoglobins 

 from different species of a genus it is found that they differ from 

 one another to a greater or less degree in angles or axial ratio, 

 in optical characters and particularly in those characters com- 

 prised under the general term " crystal habit," so that one species 

 can usually be distinguished from another by the form of its 

 haemoglobin crystals. But these differences, within the limits 

 of a given genus, are not such as to preclude the crystals from 

 all species of that genus being placed in an isomorphous series. 



A clear relationship is thus seen to subsist between the physico- 

 chemical behavior of a constituent of organisms and their place 

 in the phylogenetic scale of relationships as established by their 

 gross morphology, and a long stride has thus been taken toward 

 the establishment of a physico-chemical basis for morphological 

 distinctions. The further and entirely independent question now 

 arises, however, as to the chemical interpretation of the observed 

 physico-chemical phenomena. 



Our experience with the crystallography of inorganic and the 

 simpler organic substances has led us to infer with a considerable 

 degree of confidence that substances which show differences in 

 crystallographic structure are different chemical substances. 

 Crystal form is affected even by isomeric modifications which 

 analysis, unaided by other methods of investigation, fails to 

 reveal. Now the enormous number of atoms in a protein mole- 

 cule encourages, at first sight, the supposition that an enormous 

 and indeed, for all practical purposes, an infinite number of 

 isomerides are possible between which the most refined methods 

 of analysis would not enable us to distinguish, but which would 

 very probably differ from one another in the morphology of 

 their crystals. In point of fact, however, the available number 

 of isomers would be very greatly restricted by the necessity of 

 maintaining unaffected the amino-acid groupings of the protein 

 moiety which could not differ materially in different species 



