THE ANIMAL ANCESTRY OF MAN 6l 



that moment been held in balance? Here we come to a 

 problem with which the physiologists expect to be strugghng 

 for a long time to come. 



Fig 3. Arrangement of muscle fibers and muscle septa (myosepta) adherent 



to inner side of skin of a modern shark. 



(From Gregory, Proc. Amer. Pbilos. Soc.) 



Meanwhile we may emphasize the fact that from the 

 evolutionary viewpoint man has inherited the striped 

 muscle fiber, which is the smaller unit of his entire locomotor 

 system, from the very oldest vertebrates, and that a large 

 part of the human nervous system, like that of other verte- 

 brates, is concerned with the regulation of the locomotor 

 organs and with their effective coordination with other 

 major systems. 



The known record of fossihzed remains shows that the 

 ostracoderms, which were the immediate forerunners of the 

 vertebrates, were already in existence in the Ordovician 

 and Silurian periods, perhaps half a bilHon years distant 

 from the present day. Even at that inconceivably remote 

 epoch the most fundamental problem of vertebrate evolution 

 had already been solved and with regard to the ground-plan 

 of their anatomy the ostracoderms were actually far nearer 

 to man than they were to the one-celled starting point of life. 

 For these fish-Iike chordates were already bilaterally sym- 

 metrical, with head and tail and the ability to move in a for- 

 ward direction. In their heads they had paired sense organs 

 representing the senses of smell, vision and balance, while 

 the main divisions of their brains, as shown by study of their 



