CAUSES OF EVOLUTION 245 



on all the actions, reactions, and interactions of the body, but 

 it does not help to explain the definite origin of new characters 

 which cannot enter into "organic selection" before they exist. 

 Nor is there any evidence that while adapting itself to one 

 mode of life fortuitous variations in the heredity-chromatin for 

 every other mode of life are occurring. 



Theoretic Causes of Evolution in Mammals 



We have thus far described only the modes of evolution and 

 said nothing of the causes. In speculating on the causes of 

 character evolution in the mammals, in comparison with similar 

 body forms and characters in the lower vertebrates and even 

 in the invertebrates, it is very important to keep in mind the 

 preceding evidence that mammalian heredity-chromatin may 

 preserve all the useful functional and structural properties of 

 action, reaction, and interaction which have accumulated in 

 the long series of ancestral life forms from the protozoan and 

 even the bacterial stage. 



Since structurally the mammalian embryo passes through 

 primitive protozoan (single-celled) and metazoan (many-celled) 

 phases, it is probable that chemically it passes through the 

 same. The heredity-chromatin even in the development of 

 the highest mammals still recalls primitive stages in the devel- 

 opment of the fishes, for example, the gill-arch structure at 

 the side of the throat, which through change of function serves 

 to form the primary cartilaginous jaws (Meckelian cartilages) 

 of mammals as well as the bony ossicles which are connected 

 with the auditory function of the middle ear (Reichert's 

 theory). Similarly profound structural ancestral phases in 

 protozoan, fish, and reptile structure pervade every part of the 

 mammalian body. In race evolution there may be changes of 

 adaptation as in the law of change of function {Prinzip des Funk- 



