THE CONTINUITY OF LIFE 17 



establishment of a group which becomes more and more 

 aquatic, as represented by the divergent line leading to 34, 

 until finally a completely aquatic fish-eating animal or group 

 of animals is the result, the form at the end of the line, 34, 

 representing the highest point of specialization attained. 



The remaining descendants of 29, continuing to live in pre- 

 cisely the same habitat as their ancestors, as is here indicated 



15 



FIG. 6. Hypothetical tree illustrating the interrelations of organisms. 



Extinct forms are represented by open circles, living forms by solid black ones. 

 The same number distinguished by exponent letters signifies a close relationship. 

 The dotted areas suggest some special environment, the inhabitants of which show 

 " adaptive resemblance " although representing several unrelated lines. 



by the continuance of the line 29-35, m the same direction as 

 28-29, either remain exactly as their ancestors, or probably 

 become more highly specialized in the same direction. Con- 



