INTRODUCTION ii 



THE HEART AND LUNGS 



The heart and the kings afford important evidence of vertebrate relation- 

 ship and evolution. The increase in specialization of the respiratory appara- 

 tus, from the gill stages of the fish to the conditions of the mammalian 

 puhrionary organs, is a most rehable means of distinction between classes 

 and orders. 



Embryogenesis as Evidence of Evolutionary Kinship 



And iinally, embryogenesis, coming as a cuhiiination of reproductive 

 activity, discloses the fact that all vertebrates — fishes, amphibians, reptiles, 

 birds and mammals — however different their form, habitat, mode of Ufe 

 and behavior — arc cast in a structural mold of development hiid down in 

 accord with a common plan. 



Each department of structural organization in the body contributes 

 adequate testimony of a common ancestry. But no one of them portrays 

 more than a single phase of vertebrate adaptation. Thus the blood repre- 

 sents the metaboHc and biochemical adjustment of the animal. The osseous 

 system and the muscles are indicative only of motor and locomotor capabili- 

 ties. The teeth reveal the feeding habits and part of the protective mecha- 

 nism. The genito-urinary system bears witness to the variety of excretory 

 differentiation and the type of reproductive specialization. Embryogenesis 

 summarizes the process of structural unfolding and thus discloses the general 

 morphological plan of organization. 



The Brain as the Most Comprehensive Record 

 OF AN Evolutional Process 



The reasons why the brain contains the most comprehensive record 

 of the evolutional process are readily perceived. As an organ its mllucnces 

 pervade and dominate all other systems of the body. It is the great trans- 



