THE ORIGIN OF SPECIES 367 



5. Physiology 



Fundamental structural similarities throughout a series 

 of organisms implies fundamental physiological similarities 

 structure and function go hand in hand, each being an expres- 

 sion of -the other. But the physiological evidence is less 

 readily presented in brief form, so we may confine attention 

 to one striking example on the borderline. 



It has been known for a long time that there are important 

 chemical differences not determinable by ordinary chemical 

 analysis between the blood even of closely related species, 

 because the transfusion of the blood of one species into 

 another is usually attended by physiological disturbances 

 and often by death. It has been found by innumerable 

 transfusions and also by so-called precipitation tests of 

 the blood in vitro, that is outside the body, that the degree 

 of the ' reaction ' is in many cases proportional to the degree 

 of relationship of the species involved, as indicated by their 

 classification on the basis of anatomical criteria. 



Thus, as one would expect, human blood shows closer 

 chemical relationships with the blood of the Man-like Apes 

 than it does with that of the Old World Monkeys; closer 

 relationships with the blood of the latter than it does with 

 that of the New World Monkeys; and closer with the blood of 

 these than with that of the Lemurs; and so on. Or, descend- 

 ing to the Reptiles: paleontology indicates that there is a 

 close relationship between Lizards and Snakes and also 

 between Turtles and Crocodiles, while the reptilian ancestor 

 of the Birds was probably more closely allied with the latter 

 than the former groups. These same relationships are 

 indicated by blood tests. 



Thus aside from a few startling exceptions, which further 

 study perhaps may bring into line, all the data warrant the 

 conclusion that the chemical similarities of the blood are 



