Chemical Basis of Genus and Species 65 



As far as the genus is concerned it was found that 

 the hemoglobin crystals of any genus are isomorphous. 



In some cases this isomorphism may be extended to 

 include several genera, but this is not usually the case, 

 unless as in the case of dogs and foxes, for example, the 

 genera are very closely related. 



The most important question for us is the following: 

 Are the differences between the corresponding hemo- 

 globin crystals of different species of the same genus 

 such as to warrant the statement that they indicate 

 chemical differences? If this were the case we might 

 say that blood reactions as well as hemoglobin crystals 

 indicate that differences in the constitution of proteins 

 determine the species specificity and, perhaps, also 

 species heredity. The following sentences by Reichert 

 and Brown seem to indicate that this may be true for 

 the crystals of hemoglobin. 



The hemoglobins of any species are definite substances 

 for that species. But upon comparing the corresponding 

 substances (hemoglobins) 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 the hemoglobins crystallize in isomor- 

 phous 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 

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