326 SUMMARY AND CONCLUSIONS. 



widely separated species is hardly more than the variations that occur in 

 the different varieties of the domestic dogs. In the crystals from the cats, 

 also, we find an isomorphous group of an equally remarkable character. 

 In this case either the oxyhemoglobin crystals or the reduced hemoglobin 

 crystals may be compared, and even the oxyhemoglobin crystals with the 

 reduced-hemoglobin crystals. But while they show axial ratios that closely 

 resemble each other, the forms of the crystals that develop do not always 

 look alike. In the cats, too, we see an instance, common among minerals, 

 where different species have crystals that show different prisms or pyra- 

 mids, which while having the same axial ratio do not have the same form, 

 but are multiples the one of the other. Both the dogs and the cats have 

 hemoglobins that crystallize in the orthorhombic system, but with very 

 different axial ratios. 



Perhaps a more remarkable instance of isomorphism is exhibited in 

 the three species of bears examined, Ursus americanus, Ursus maritimus, 

 and Melursus ursinus. These vary considerably in axial ratio, but the two 

 species of Ursus are close in ratio of a: 6 and in prism angle; all three, how- 

 ever, belong to the monoclinic sphenoidal or monoclinic hemimorphic group, 

 and all three have a very remarkable habit of twinning on a prism of about 

 60. The isomorphism, due to the differences of development of the crys- 

 tals, sometimes appears to be only partial, but in the case of these three 

 bears it will be seen that the extinction angles are all very nearly alike. 

 Thus the angle c A a, the extinction angle in the crystals of these three 

 species, is 19 in the black bear and 20 in each of the others. 



From this isomorphism of the crystals of any genus we may infer a 

 correspondence of structure of the molecules of the homologous hemoglobins 

 derived from that genus. 



Constancy and specificity of the crystallographic characters of individual 

 species : 



The oxyhemoglobin obtained from the same blood crystallizes in the 

 same form, with the same axial ratio, though often with different habit, 

 when obtained by different methods of preparation. When several forms 

 exist, each form, a-oxyhemoglobin, /^-oxyhemoglobin, etc., appears always in 

 its own proper form and axial ratio when the bloods of different individuals 

 of the species are examined. The same is true of the other hemoglobins 

 me toxy hemoglobin, reduced hemoglobin, methemoglobin ; so that the hemo- 

 globins of any species are definite substances for that species. But upon 

 comparing the corresponding substances in different species of a genus it 

 is generally found that they differ the one from the other to a greater or 

 less degree ; the differences being such that when complete crystallographic 

 data are available the different species can be distinguished by these 

 differences in their hemoglobins. As these hemoglobins crystallize in iso- 

 morphous series, the differences between the angles of the crystals of the 

 species of a genus are not, as a rule, great; but they are as great as is usually 

 found to be the case with minerals or chemical salts that belong to an isomor- 

 phous group. How much the crystals of the corresponding hemoglobins 

 vary from each other in different species of a genus may be seen by reference 



