PART II. THE EVOLUTION OF ANIMAL FORM 



CHAPTER IV 



THE ORIGINS OF ANIMAL LIFE AND EVOLUTION 

 OF THE INVERTEBRATES 



Evolution of single-celled animals or Protozoa. Evolution of many-celled 

 animals or Metazoa. Pre-Cambrian and Cambrian forms of Inverte- 

 brates. Reactions to climatic and other environmental changes of geo- 

 logic time. The mutations of Waagen. 



A prime biochemical characteristic in the origin of animal 

 life is the derivation of energy neither directly from the water, 

 from the earth, nor from the earth's or sun's heat, as in the 

 most primitive bacterial stages; nor from sunshine, as in the 

 chlorophyllic stage of plant life; but from its stored form in 

 the bacterial and plant world. All animal life is chemically 

 dependent upon bacterial and plant life. 



Many of the single-celled animals like the single-celled bac- 

 teria and plants appear to act, react, and interact directly 

 with their lifeless and life environment, their protoplasm be- 

 ing relatively so simple. We do not know how far this action, 

 reaction, and interaction affects the protoplasm only, and how 

 far it affects both protoplasm and chromatin. It would seem 

 as if even at this early stage of evolution the organism-proto- 

 plasm was sensitive while the heredity- chromatin was relatively 

 insensitive to environment, stable, and as capable of conserving 

 and reproducing hereditary characters true to type as in the 

 many-celled animals in which the heredity-chromatin is deeply 

 buried within the tissues of the organism remote from direct 

 environmental reactions. 



