THEORY OF EVOLUTION 505 



primitive chordates, but is present as such in only the embryo stage 

 of all Others. The development of the heart from the tubular con- 

 dition through the two-chambered, three-chambered, and finally four- 

 chambered condition, illustrates the same progressive development. 



Physiological Evidence.— The fact that all protoplasm possesses 

 the same set of fundamental properties or capacities as contractility, 

 irritability, metabolism, etc., is in itself a definite indication of rela- 

 tionship of all organisms, since they are all composed of protoplasm. 

 Too, all protoplasm acts under similar lav7S and conditions. 



Such natural substances of animal bodies as the hormones, or anti- 

 bodies or even enzymes are almost universal in their reactions among 

 chordates and even among nonchordates. Most of them are inter- 

 changeable from one animal group to another. A deficiency of pep- 

 sin or adrenalin in man may be supplied from a cow, a hog, a cat, 

 a rat, or a dog. The blood of all vertebrates has certain physiological 

 similarities and some specific differences. The blood of large groups of 

 human beings will mix without agglutination and is said to ''match." 

 Other individuals' blood may not ''match" in this type but will mix 

 with blood from another group. The agglutination (clumping of red 

 corpuscles) when blood from two individuals is brought together is 

 due to the reaction of two substances produced by the corpuscles in 

 one or both of the blood samples. The designation of the blood 

 groups depends upon the presence or absence of one or both of these 

 substances. One blood group (0) contains neither, another (A) 

 contains one substance, a third (B) the second substance only, while 

 a fourth group (AB) contains both substances. Serum of the first 

 group will agglutinate corpuscles when mixed with any of the other 

 three. The groups with the single substance will agglutinate cor- 

 puscles in blood having only the other substance or blood with both 

 substances. The serum from blood with both substances will not 

 cause agglutination in any of the others. 



In lower mammals some similar blood groups have been found, 

 but it is only in the apes that the groups correspond to those in the 

 human being. This is an indication of the rather close relationship 

 of these animal groups. In the human being and in other animals, 

 this blood characteristic is permanent in the individual, and it is 

 hereditary. 



Serum (blood minus corpuscles and fibrinogen) studies also estab- 

 lish certain relationships among animals. If small quantities of 



