74 RALPH S. LILLIE. 



chemical specificity is primarily the property of the proteins. 

 Other biochemical compounds appear to be chemically the same 

 wherever found, but the proteins vary in their specific character 

 from species to species. Moreover, physiologically correspond- 

 ing or "homologous" proteins are more nearly alike in their 

 chemical and physical characters the more closely related the 

 species are from which they are taken. There is thus a general 

 parallelism between the degree of chemical relationship exhibited 

 by homologous proteins from different organisms, and the degree 

 of biological relationship existing between the species. The 

 indications of this are too various to present in detail in this 

 brief paper, and the evidence has recently been reviewed in an 

 admirable manner by Loeb in his "Organism as a Whole." 1 

 The specificity which such proteins exhibit when used as anti- 

 gens, e. g., in the formation of precipitins or specific cytolysins, 

 or in the phenomena of anaphylaxis, shows clearly that the pio- 

 teins of one species are chemically distinct from the corresponding 

 proteins of even nearly related species, and still more distinct 

 from those of more distant species. Nuttall's well-known work 

 shows that the ability of a given precipitin to react with and 

 precipitate its corresponding protein from another species is a 

 close indication of the degree of blood-relationship between the 

 species under consideration. 2 



Apart from these facts, whose significance in relation to the 

 present problem is now well recognized, there are other evidences 

 of chemical specificity in proteins that offer clearer indica- 

 tions of the nature of the connection between the chemical char- 

 acter of a protein and the character of the structures which it 

 forms in the living cell. The work of Reichert and Brown has 

 shown that heemoglobin crystals from a given species exhibit 

 form-characters which are definite and specific for the species. 

 This means that when the protein separates from solution in the 

 process of crystallization the molecules, as they unite to form 

 larger crystalline aggregates, by degrees build up structures with 

 definite form-characters the typical forms of one species exhib- 

 iting constant differences from those of other species, even of 



1 Cf. Chapter 3: "The Chemical Basis of Genus and Species." 



2 Nuttall, "Blood Immunity and Blood Relationships." Cambridge University 

 Press, 1904. 



