10 INTRODUCTION 



porpoises) are more closely allied to cud-chewing animals (ruminants) 

 and pigs. 



Blood reactions indicate that a strong interrelationship characterizes 

 all reptilian forms, although there is some serial grading in these afiinities as 

 among turtles, crocodiles, lizards and snakes respectively. 



Avian blood tests show a striking similarity in the hemal constituency 

 of all birds. This fact stands out in contrast to the mammalian blood which 

 has a specific variability in the intensity of its reactions dependent upon the 

 order and family of the animals tested. 



In general, the precipitin blood tests are among the most convincing 

 proofs put forward in support of the evolutionary hypothesis. 



THE BONY SYSTEM 



The bony system of the body (Fig. 4) is also an illuminating recorder 

 of evolutional kinship. The skeleton of the fore- and hindlimbs sheds much 

 light on the modifications connected with the locomotor apparatus. In accord- 

 ance with the limb specialization as fins, paddles, wings, hoofs, paws, claws, 

 hands or feet, the general propinquities in kinship may to a certain extent be 

 estimated. The size and shape of the skull are equally significant in deter- 

 mining an exact classilication of species. 



THE MUSCULAR SYSTEM 



Another significant index of a generalized ground-plan in vertebrate 

 organization is the muscular system. This system, while it demonstrates 

 the great stability underlying the motor apparatus, discloses a flexibility 

 capable of providing for the special needs of aquatic, amphibian, terrestrial 

 and aerial life. It thus reveals propinquities in kinship amongst animals 

 which have adjusted their locomotor apparatus to similar or closely allied 

 conditions of environment. 



