296 BIOLOGY: GENERAL AND MEDICAL 



may explain the difficulties attending successful hybridi- 

 zation. The germinal cells of specifically different 

 organisms doubtless possess the same sensitivity to 

 heterologous cells that are manifested by the somatic 

 cells, so that, instead of fertilizing one another, they 

 remain indifferent to one another, repel one another, or 

 perhaps even destroy one another. 



Finally, blood relationship has a distinct bearing 

 upon the subject of symbiosis for organisms whose 

 chemical affinities are opposed to one another may be 

 unable to become symbionts, and in cases of parasitism, 

 it is conceivable that one of the first adaptations to be 

 acquired by the parasite is tolerance to the physiologico- 

 chemically antagonistic conditons to be found in the 

 body of the host. Indeed, as will be shown in the chap- 

 ter upon Infection and Immunity, the experimental 

 exaltation of microorganismal virulence is a matter of 

 overcoming the body defenses, which from the present 

 standpoint may be looked upon as the establishment of 

 a tolerance toward originally antagonistic chemico- 

 physiological conditions. 



REFERENCES. 



GEORGE H. F. NUTTALL: " Blood Immunity and Blood Relation- 

 ship," Cambridge, 1904. 



EDWARD TYSON REICHERT and AMOS PEASLEE BROWN: "The 

 Differentiation and Specificity of Corresponding Proteins 

 and Other Vital Substances in Relation to Biological 

 Classification and Organic Evolution: The Crystal- 

 lography of Hemoglobins." The Carnegie Institution, 

 Washington, 1909. 



MAGNUS and FRIEDENTHAL: "Ueber die Specifitat der Verwand- 

 schaftsreaction der Pflanzen." Berichte der deutschen 

 botanischen Gesellschaft, xxv, 1907, 242; xxiv, 1906, 

 601. 



