A DEFINITION OF EVOLUTION 



ferences, comparable to the differences of structure with which compara- 

 tive anatomy deals. 



HEMATOLOGICAL CHARACTERISTICS 



If a drop of blood from a species having hemoglobin as its respiratory 

 pigment is treated in the proper way, crystals of hematin can be obtained. 

 The details of crystal structure differ from species to species, and they 

 parallel classification in a remarkable way. Crystals obtained from all of 

 the species of a genus share many characteristics, while crystals from 

 members of different classes have characteristics which are mutually ex- 

 clusive. Thus it is possible, on the basis of hematin crystals alone, to 

 distinguish between the various classes of vertebrates. 



Comparative Serology. Much the most impressive physiological evi- 

 dence is chawn from the field of comparative serology. If a small amount 

 of the blood serum of any animal is injected into a guinea pig (or other 

 test animal), the foreign blood acts as an antigen, that is, it causes the 

 production in the serum of the guinea pig of aniihodics which will pre- 

 cipitate and destroy the antigen if a second inoculation should occur. The 



PROCEDURE 



THE REAGENTS THE TEST 



Q 



ANTIGEN 



CSERUH^ 



s 



ANTISERUM 



AN ACTUAL 



A. HOMOLOGOUS TEST 



EXPERIMENT 



toe* !»«» !»«• »«eo «ooe 4oes «••• <• 



BEEF SERUM 

 AND 

 ANTI-BEEF SERUM 



WWW 



U u t 



w y 



HETEROLOGOUS TEST 



■ »00 !)«• (••• !••• a««« «««« «,«« f»1»l>\ 



SHEEP SCRUM 

 AND 

 ANTI-BEEF SERUM 



W 'CJ' 



W 



Q 



C. RELATIONSHIP OF 



BEEF TO SHEEP = ^f^ = «« 7> 



FiounK 22. PnocKDUHF IN Making A RiNG Tivs I' lou Si:iu)lo(;ic:al Relationship. 

 Uncliliitt'd aiitisrniiH is layered iiiulcr diluted antigen in saline solution. A white ring 

 of precipitate indicates a positiNc reaction. The titer is the liighest dilution of the 

 antigen which will give a positive nuiction. ( From Boyden, Fliysiol. 'Zool., V. 15, 1942.) 



56 



