POPULAR SCIENCE 



79 



from such a type. In the one direction the great toe might be 

 made movable and opposable as it is in most cases ; or it 

 might be lost completely or in part ; or it might grow and keep 

 close to the toes, and so assume the human type. But in 

 accordance with Dollo's generalisation on the irreversibility of 

 the course of evolution it is unlikely that the type should change 

 from rigidity in the ancestral ground ape to mobility in the 

 gorilla, and back again to rigidity in man. Life on the ground 



Fig. 3. 



Left foot and hand of Phenacodus prlmzvus. 



would be as fatal to a projecting great toe as it is assumed to 

 have been to the ancestral tail. 



3. Passing now from structure to function we select instinct 

 as a well-marked line of demarcation between man and the 

 apes. Its absence in man is correlated with the great brain 

 and with the absence of any structures elaborated into weapons 

 of attack or defence, such as the teeth of the gorilla or the 

 ears of the chimpanzee. By some means the brain of man 

 has, during its period of super-growth, been able to jorget the 

 impressions which it inherited from the pre-human stage of 

 existence. The cells of the embryonic tail and vermiform 

 appendix still faithfully appear as they have done for millions 

 of years. But the cells of the brain have changed and are 

 different from their ancestors' perhaps only a few hundred thou- 

 sand years ago. This is another special human phenomenon. 



While it is probable that instinct in birds and mammals 

 was not as well marked in the Eocene as it is at the present 



