170 THE BIDDLE OF THE UNIVEESE. 



millions of years, it conducts to that marvellous 

 structure of the human brain which seems to entitle 

 the highest primate form to quite an exceptional 

 position in nature. Since a clear conception of this 

 slow and steady progress of our phyletic psychogeny 

 is indispensable for a true psychology, we must divide 

 that vast period into a number of stages or sections : 

 in each of them the perfecting of the structure of the 

 nervous centre has been accompanied by a correspond- 

 ing evolution of its function, the psyche. I distin- 

 guish eight of these periods in the phylogeny of 

 the spinal cord, which are characterised by eight 

 different groups of vertebrates : — (1) the acrania ; (2) 

 the cyclostomata ; (3) the fishes ; (4) the amphibia ; 

 (5) the implacental mammals (monotremes and marsu- 

 pials) ; (6) the earlier placental mammals, especially 

 the prosmiiae ; (7) the younger primates, the simiae ; 

 and (8) the anthropoid apes and man. 



I. First stage — the acrania: their only modern 

 representative is the lancelot or amphioxus ; the 

 psychic organ remains a simple medullary tube, and 

 contains a regularly segmented spinal cord, without 

 brain. 



II. Second stage — the cyclostomata : the oldest group 

 of the craniota, now only represented by the petromy- 

 zontes and myxiaoides : the fore-termination of the 

 cord expands into a vesicle, which then subdivides 

 into five successive parts — the great-brain, inter- 

 mediate-brain, middle-brain, little-brain, and hind- 

 brain : these five cerebral vesicles form the common 

 type from which the brain of all craniota has evolved, 

 from the lamprey to man. 



III. Third stage — the primitive fishes fselachii) : 

 similar to the modern shark : in these oldest 



