4 SYMBIONTICISM AND THE ORIGIN OF SPECIES 



of species. It is the factor that causes permanent variations 

 in the structure of animals and plants. 



Another unknown factor of evolution is the principle 

 determining the direction of organic evolution, or the goal 

 toward which evolution is proceeding. This factor has 



/ ^, 



Fig. 1 



been recognized, but very httle has been attempted or 

 accomplished in determining its nature. It is possible that 

 this particular feature of organic evolution has been and 

 still is a stumbling block to the layman's appreciation of 

 the reality of organic evolution. From a common ancestor 



