24 UNIVEESALITY OF DEGENERATIVE EVOLUTION 



through which they passed during their ancestral 

 evolution. Generally speaking, and especially in 

 the case of plants, the development of organs in 

 an individual is direct, and gives no clue to its 

 ancestral history. Moreover, when there is a re- 

 capitulation of ancestral stages, it often happens 

 that evolution takes place without leaving traces 



n of the various 



stages. This is 

 especially the 

 case in complex 

 organs which 

 have been pro- 

 duced by many 

 lines of evolu- 

 tion converging 

 ^ in a single struc- 

 ture a struc- 

 ture which thus 

 becomes the seat 

 func- 



b a 



FIG. 1. Diagram showing the evolution of pyramidal n f 

 cells in the animal series. r 



The upper series of cells represents the psychic cell in tion Or Set of 

 various vertebrates: A, the frog; B, the lizard; 

 C, the rat; Z>, man. The lower series shows the lUIlCtlOnS. 

 progressive stages in the evolution of the pyramidal 



cell in the human brain : a, the neuroblast without I he neuron, 

 protoplasmic processes; 6, the appearance of the 



nerve process and of the terminal ramifications ; for instance, the 

 c, the nerve more fully developed ; rf, appearance of . 



lateral branches of the axis cylinder ; e, development gangllOniC Cell 

 of protoplasmic outgrowths of the protoplasm of 

 nerve-cell and nerve. (Ramon y Cajal.) of the COrteX Of 



the human brain, passes successively through stages 

 corresponding to those which are to be found in the 

 adult fish, frog, bird, and mammal. In this case 



