PHYLOGENETIC RELATIONS OF ANIM.VL GROUPS 883 



The course of development and the modifications shown in the 

 aortic arches or main arteries leaving the heart and passing through 

 the gill regions is a specific example of the manner in which a spe- 

 cific set of structures follows out a repetition of ancestral stages in 

 the development of the individual. Fig. 445 shows a comparison 

 of the arrangement of the branchial (aortic) arches from the primi- 

 tive set of six through the fishes, Amphibia, reptiles, to the birds 

 and mammals, with a modified condition of three arches. 



Along with the aortic arch situation are other examples of similar 

 stages of development in all forms of the group. Gills are present 

 in all chordates at some time. In the primitive ones the gills are 

 functional throughout life ; in the more advanced types they are 

 only transitory gill structures. In frogs and toads, the tadpole stage 

 is essentially fishlike, and they become amphibian at metamorphosis. 

 The notochord is a definite structure throughout the life of the 

 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 laws and conditions. 



Such natural substances of animal bodies as the hormones, or 

 antibodies or even enzymes are almost universal in their reactions 

 among chordates and even among nonchordates. Most of them are 

 interchangeable from one animal group to another. A deficiency 

 of pepsin 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 precipitation and 

 is said to "match." Other individuals' blood may not "match" 

 in this type but will mix with blood from another group. The ag- 

 glutination (clumping of red corpuscles) which occurs when samples 

 of blood from two such individuals are put 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 group depends 

 upon the presence or absence of one or both of these substances. One 



